CHAPTER XXI
.
FOSSIL FERNS.
=Osmundaceae.=
From the Culm of Silesia, Stur[761] described impressions of sterile fronds which he named _Todea Lipoldi_ on the ground of the similarity of the finely divided pinnules to those of _Todea superba_ and other filmy species of the genus. The type-specimen of Stur (in the Geological Survey Museum, Vienna) affords no information as to sporangial characters and cannot be accepted as an authentic record of a Lower Carboniferous representative of the family. Another more satisfactory but hardly convincing piece of evidence bearing on the presence of Osmundaceae in pre-Permian floras has been adduced by Renault[762], who described petrified sporangia from the Culm beds of Esnost in France as _Todeopsis primaeva_ (fig. 256, F). These pyriform sporangia are characterised by the presence of a plate of large cells comparable with the subapical group of “annulus” cells in the sporangia of the recent species (fig. 221).
Zeiller[763] has published a figure of some sporangia described by Renault from Autun resembling the Osmundaceous type in having a plate of thick-walled cells instead of a true annulus, but the plate is larger than the group of cells in the recent sporangia, and both sporangia and spores are smaller in the fossil. The sporangia from Carboniferous rocks described by Weiss as _Sturiella_[764] bear some resemblance to those of recent Osmundaceae, but there is no adequate reason for referring them to this family.
The generic name _Pteridotheca_ is employed by Scott as a convenient designation for unassigned petrified sporangia of Palaeozoic age with an annulus and other characters indicating fern-affinity. In the species _P. Butterworthi_[765] the sporangia are characterised by a group of large cells suggesting comparison with the annulus, or what represents the annulus, in Osmundaceae and Marattiaceae. Scott has also described a sporangium from the Coal-Measures containing germinating spores[766]; the structure is similar to that of recent Osmundaceous sporangia, and it is interesting to note that germinating spores have been observed in the recent species _Todea hymenophylloides_[767].
Additional evidence of the same kind is afforded by fertile specimens of a quadripinnate fern with deeply dissected oval-lanceolate pinnules described by Zeiller from the Coal-Measures of Heraclea in Asia Minor as _Kidstonia heracleensis_[768] (fig. 256, E). Carbonised sporangia were found at the base of narrow lobes of the ultimate segments and, as seen in fig. 256, E, the sporangial wall is distinguished by a plate of larger cells occupying a position like that of the “annulus” of recent Osmundaceae. Zeiller regards the sporangia as intermediate between those of Osmundaceae and Schizaeaceae. From the same locality Zeiller describes another frond bearing somewhat similar sporangia as _Sphenopteris_ (_Discopteris_) _Rallii_ (fig. 256, D)[769]: the term _Discopteris_ was instituted by Stur for fertile fronds referred by him to the Marattiaceae[770].
It is by no means safe to assume that these and such Upper Carboniferous sporangia as Bower[771] compared with those of _Todea_ were borne on plants possessing the anatomical characters of Osmundaceae rather than those of the extinct Palaeozoic family Botryopterideae. This brings us to the important fact, first pointed out by Renault, that the Botryopterideae are essentially generalised ferns exhibiting many points of contact with the Osmundaceae[772]. It is clear that whether or not we are justified in tracing the Osmundaceae as far back as the Lower Carboniferous period, some of the characteristics of the family were already foreshadowed in rocks of this age.
Through a fortunate accident of preservation, unequivocal evidence of the existence of Osmundaceae in the Palaeozoic era is supplied by the Russian Upper Permian genera _Zalesskya_ and _Thamnopteris_.
_Zalesskya._
This generic title has been instituted by Kidston and Gwynne-Vaughan[773] for two Russian stems of Upper Permian age, one of which was named by Eichwald[774] _Chelepteris gracilis_, but the probability that the type of the genus _Chelepteris_ is generically distinct from Eichwald’s species necessitated a new designation for the Permian fern.
In habit the stem of _Zalesskya_ resembles that of an _Osmunda_ or a _Todea_, but it differs in the possession of a stele composed of a continuous cylinder or solid column of xylem surrounded by phloem, and by the differentiation of the xylem into two concentric zones. The leaves are represented by petiole-bases only; the sporangia are unknown. The stem and leaf-base anatomy fully justifies the inclusion of _Zalesskya_ in the Osmundaceae.
_Zalesskya gracilis_ (Eichwald). Fig. 248.
The type-specimen is a partially decorticated stem, from Upper Permian beds in Russia, provided with a single stele, 13 mm. in diameter, surrounded by a broad thin-walled inner cortex containing numerous leaf-traces and occasional roots: this was doubtless succeeded by a sclerotic outer cortex. In its main features _Zalesskya gracilis_ agrees closely with _Z. diploxylon_ represented in fig. 249. The stele consists of a continuous cylinder of xylem exhibiting a fairly distinct differentiation into two zones, (i) a broader outer zone of narrower scalariform tracheae (_x ii_, fig. 248) in which 20 to 25 protoxylem strands (_px_) occur just within the edge, (ii) an inner zone of broader and shorter tracheae (fig. 248, _x i_). The protoxylem elements (_px_, fig. 248) are characterised by a single series of scalariform pits, while the metaxylem elements have multiseriate pits like those on the water-conducting elements of recent Osmundaceae. The tracheae show an interesting histological character in the absence of the middle substance of their walls, a feature recognised by Gwynne-Vaughan[775] in many recent ferns. External to the xylem and separated from it by a parenchymatous sheath is a ring of phloem, _ph_, composed of large sieve-tubes and parenchyma separated from the inner cortex by a pericycle 4 to 5 layers in breadth. The occurrence of a few sclerotic cells beyond the broad inner cortex points to the former existence of a thick-walled outer cortex. The leaf-traces are given off as mesarch strands from the edge of the xylem; they begin as prominences opposite the protoxylem and become gradually detached as xylem bundles, at first oblong in transverse section, then assuming a slightly crescentic and reniform shape, while the mesarch protoxylem strand takes up an endarch position. As a trace passes further out the curvature increases and the protoxylem strands undergo repeated bifurcation; it assumes in fact the form and general type of structure met with in the leaf-traces of _Todea_ and _Osmunda_. Numerous diarch roots, given off from the stele at points just below the outgoing leaf-traces, pass outwards in a sinuous horizontal course through the cortex of the stem.
[Illustration: FIG. 248. _Zalesskya gracilis_ (Eich.). Transverse section of part of the stele: _ph_, phloem; _x i_, _x ii_, xylem; _px_, protoxylem. (After Kidston and Gwynne-Vaughan. × 20.)]
[Illustration: FIG. 249. _Zalesskya diploxylon_. Kidston and Gwynne-Vaughan. Transverse section of stem. _ph_, phloem. (After Kidston and Gwynne-Vaughan. × 2½.)]
In _Zalesskya gracilis_ the xylem cylinder was probably wider in the living plant than in the petrified stem. In _Zalesskya diploxylon_[776], in all probability from the same Russian locality, there can be little doubt that the xylem was originally solid to the centre (fig. 249). In this species also the phloem forms a continuous band (_ph_, fig. 249) consisting of four to six layers of sieve-tubes.
_Thamnopteris_.
_Thamnopteris Schlechtendalii_ (Eich.). Figs. 250, 312, A, Frontispiece.
In 1849 Brongniart[777] proposed the name _Thamnopteris_ for a species of fern from the Upper Permian of Russia originally described by Eichwald as _Anomopteris Schlechtendalii_. A new name was employed by Brongniart on the ground that the fossil was not generically identical with the species previously named by him _Anomopteris Mougeotii_[778]. Eichwald’s specimen has been thoroughly investigated by Kidston and Gwynne-Vaughan[779]. The stem (Frontispiece) agrees in habit with those of _Zalesskya_ and recent Osmundaceae; on the exposed leaf-bases the action of the weather has etched out the horse-shoe form of the vascular strands and laid bare numerous branched roots boring their way through the petiole stumps. The centre of the stem is occupied by a protostele 13 mm. in diameter consisting of solid xylem separated by a parenchymatous sheath from a cylinder of phloem. The xylem is composed mainly of an axial column of short and broad reticulately pitted tracheae (fig. 250, _b_ and Frontispiece), distinguished from the sharply contrasted peripheral zone of normal scalariform elements, _a_, by their thinner walls and more irregular shape. The protoxylem, _px_, is represented by groups of narrower elements rather deeply immersed in the peripheral part of the metaxylem. A many-layered pericycle, _per_, and traces of an endodermis, _en_, succeed the phloem, _ph_, which is characterised by several rows of large contiguous sieve-tubes; beyond the endodermis is a broad thin-walled inner cortex. The leaf-traces arise as in _Zalesskya_, but the protoxylem in _Thamnopteris_ is at first central; as the trace passes outwards a group of parenchyma appears immediately internal to the protoxylem elements and gradually assumes the form of a bay of thin-walled tissue on the inner concave face of the curved xylem. The next stage is the repeated division of the protoxylem strand until, in the sclerotic outer cortex, the traces acquire the Osmundaceous structure (fig. 312, A, p. 453). The petiole bases have stipular wings as in _Todea_ and _Osmunda_.
[Illustration: FIG. 250. _Thamnopteris Schlechtendalii_ (Eich.). Part of stele: _a_, outer xylem; _b_, inner xylem. (After Kidston and Gwynne-Vaughan. × 13.)]
[Sidenote: OSMUNDACEAE]
The striking feature exhibited by these Permian plants is the structure of the protostele, which in _Thamnopteris_ and probably in _Zalesskya diploxylon_ consists of solid xylem surrounded by phloem: this may be regarded as the primitive form of the Osmundaceous stele. In _Osmunda regalis_ and in other recent species of the genus the xylem cylinder has the form of a lattice-work; in other words, the departure of each leaf-trace makes a gap in the xylem and the overlapping of the foliar-gaps results in the separation of the xylem into a number of distinct bundles. In _Zalesskya gracilis_ the continuity of the xylem is not broken by overlapping gaps; in this it agrees with _Lepidodendron_. In _Thamnopteris_ the centre of the stele was occupied by a peculiar form of xylem obviously ill-adapted for conduction, but probably serving for water-storage and comparable with the short and broad tracheae in _Megaloxylon_[780]. There is clearly a well-marked difference in stelar anatomy between these two Permian genera and _Todea_ and _Osmunda_: this difference appears less when viewed in the light of the facts revealed by a study of the Jurassic species _Osmundites Dunlopi_.
[Illustration: FIG. 251. _Lonchopteris virginiensis._ (After Fontaine. ½ nat. size.)]
As possible examples of Triassic Osmundaceae reference may be made to some species included in Stur’s genus _Speirocarpus_[781]. _S. virginiensis_ was originally described by Fontaine[782] from the Upper Triassic rocks of Virginia as _Lonchopteris virginiensis_ (fig. 251) and has recently been figured by Leuthardt[783] from the Keuper of Basel. The sporangia, which are scattered over the lower surface of the pinnules, are described as globose-elliptical and as having a rudimentary apical annulus; no figures have been published. In habit the frond agrees with _Todites Williamsoni_, but the lateral veins form an anastomosing system like that in the Palaeozoic genus _Lonchopteris_ (fig. 290, B). There would seem to be an _a priori_ probability of this species being a representative of the Osmundaceae and not, as Stur believed, of the Marattiaceae. Seeing that _Lonchopteris_ is a designation of a purely provisional kind, it would be convenient to institute a new generic name for Triassic species having the Lonchopteris venation, which there are good reasons for regarding as Osmundaceous ferns.
Similarly _Speirocarpus tenuifolius_ (Emmons) (= _Acrostichites tenuifolius_ Font.), which resembles _Todites Williamsoni_ (see p. 339) not only in habit and in the distribution of the sporangia but also in the venation, is probably an Osmundaceous species.
_Osmundites._
_Osmundites Dunlopi_, Kidston and Gwynne-Vaughan[784], fig. 252.
This species was found in Jurassic rocks in the Otago district of New Zealand in association with _Cladophlebis denticulata_[785] (fig. 257). The type-specimen forms part of a stem 17 mm. in diameter surrounded by a broad mass of crowded leaf-bases. The stele consists of an almost continuous xylem ring (fig. 252) enclosing a wide pith: the phloem and inner cortex are not preserved but the peripheral region of the stem is occupied by a sclerotic outer cortex. The mass of encasing leaf-bases resolves itself on closer inspection into zones of foliage-leaf petioles and the petioles of scale-leaves with an aborted lamina. A similar association of two forms of leaf is seen in the existing American species _Osmunda Claytoniana_ and _O. cinnamomea_. The cortex and armour of leaf-bases are penetrated by numerous diarch roots. The xylem cylinder, six to seven tracheae broad, is characterised by the narrower diameter of its innermost elements and—an important point—by the fact that the detachment of a leaf-trace does not break the continuity of the xylem cylinder (fig. 252). Each leaf-trace is at first elliptical in section; it then becomes curved inwards and gradually assumes the horse-shoe form as in _Zalesskya_ and in the recent species. The single endarch protoxylem becomes subdivided until in the petiole it is represented by 20 or more strands.
[Illustration: FIG. 252. _Osmundites Dunlopi_ Kidst. and G.-V. Portion of xylem showing the departure of a leaf-trace. (After Kidston and Gwynne-Vaughan; × 36.)]
In the continuity of the xylem cylinder this species of _Osmundites_ shows a closer approach to _Todea barbara_ or _T. superba_ (fig. 221, B) than to species of _Osmunda_; it differs from _Zalesskya_ in having reached a further stage in the reduction of a solid protostele to one composed of a xylem cylinder enclosing a pith. This difference is of the same kind as that which distinguishes the stele of _Lepidodendron rhodumnense_ from _L. Harcourtii_. In _Lepidodendron_ short tracheae occasionally occur on the inner edge of the xylem cylinder, and in recent species of _Todea_ the same kind of reduced tracheae are met with on the inner edge of the xylem[786]. In both cases the short tracheae are probably vestiges of an axial strand of conducting elements which in the course of evolution have been converted into parenchymatous cells. In _Lepidodendron vasculare_ the mixed parenchyma and short tracheae in the centre of the stele represent an intermediate stage in xylem reduction, and the arrangement in vertical rows of the medullary parenchyma in _Lepidodendron_ is precisely similar to that described by Kidston and Gwynne-Vaughan in _Thamnopteris_. In both cases the rows of superposed short cells have probably been produced by the transverse septation of cells which began by elongating as if to form conducting tubes and ended by assuming the form of vertical series of parenchymatous elements.
[Illustration: FIG. 253. _Osmundites Kolbei_ Sew. (⅓ nat. size.)]
In another Jurassic species, _Osmundites Gibbiana_[787], the xylem is of the _Osmunda_ type and consists of about 20 strands instead of a continuous or almost continuous cylinder.
[Illustration: FIG. 254. _Osmundites Kolbei._ (Leaf-scars.)]
_Osmundites Kolbei_ Seward, figs. 253–255.
This species was founded on a specimen obtained by Mr Kolbe from the Uitenhage series of Cape Colony[788]. The fossil flora and fauna of this series point to its correlation with the Wealden or Neocomian strata of Europe[789]. The type-specimen consists of several pieces of a stem (fig. 253) which reached a length of about 90 cm. On the weathered surface the remains of petiole-bases are clearly seen and on the reverse side of the smaller piece shown in the figure numerous sinuous roots are present in association with the leaf-stalks. The depression _c_ in the larger specimen may mark the position of a branch: at _a_ fig. 253 (enlarged in fig. 254, _a_) the vascular strand of a petiole is exposed as a broad =U=-shaped band and at _b_ (fig. 254, _b_) the form of the petiole-bases is clearly shown[790]. With the stem were found imperfectly preserved impressions of fronds referred to _Cladophlebis denticulata_, a common type of leaf which was found also in association with the slightly older New Zealand stem, _Osmundites Dunlopi_.
[Illustration: FIG. 255. _Osmundites Kolbei_ Sew. Transverse section, from a photograph supplied by Dr Kidston and Mr Gwynne-Vaughan. (2½ nat. size.)]
An examination of the internal structure of the South African stem by Dr Kidston and Mr Gwynne-Vaughan has revealed many interesting features, which will be fully described in Part IV. of their Monograph on fossil Osmundaceous stems. I am greatly indebted to these authors for allowing me to publish the following note contributed by Dr Kidston:—
“The section of _Osmundites Kolbei_ Seward, shown in fig. 255, presents the usual appearance of an Osmundaceous stock. The parts contained in this section are the stele, inner and outer cortex and a portion of the surrounding mantle of concrescent leaf-bases. The whole specimen has suffered much from pressure, but if restored to its original form the xylem ring must have been about 19 mm. in diameter. The number of xylem strands is about fifty-six and several of them are more or less joined as in the modern genus _Todea_. The tracheae are of the typical Osmundaceous type, that is to say, the pits are actual perforations and several series of them occur on each wall of the larger tracheae.
“The most interesting structural characteristic of _Osmundites Kolbei_ is not well seen in the figure owing to the compression of the xylem ring. This consists in the occurrence of tracheae in the pith. In fact, we have here a mixed pith, composed of parenchyma and true tracheae, a condition which connects the _Osmundaceae_ with a parenchymatous medulla with those possessing a solid xylem stele like _Zalesskya_ and _Thamnopteris_ and so completes the series of transitions extending from the older and solid-steled forms to the modern medullated members of the _Osmundaceae_.”
_Osmundites skidegatensis_, Penhallow.
This lower Cretaceous Canadian species, first described by Penhallow[791] and more recently by Kidston and Gwynne-Vaughan[792], is remarkable for the large size of the stem, the stele alone having a diameter of 2·4 cm. Penhallow figures a fragment of a leaf bearing a superficial resemblance to that of _Osmunda Claytoniana_, which may be the foliage borne by _Osmundites skidegatensis_. The xylem cylinder is broken by the exit of leaf-traces into 50 or more strands varying in size and shape, and it is noteworthy that the phloem is also interrupted as each leaf-trace is given off. In recent species the xylem cylinder is almost always interrupted, but the phloem retains its continuity. In the Canadian fossil an internal band of phloem occurs between the xylem and the pith, and this joins the external phloem at each leaf-gap. This internal phloem finds an interesting parallel in certain recent species[793], but in these the internal and external phloem do not meet at the foliar gaps as they do in the extinct type. In _Osmunda cinnamomea_ the internal phloem occurs only at the regions of branching of the stem stele; in the fossil it is always present.
It is clear that _Osmundites skidegatensis_ represents the most complex type of stem so far recognised in the Osmundaceae; it illustrates a stage in elaboration of the primitive protostele in advance of that reached by any existing species.
• • • • •
The primitive Osmundaceous stele was composed of solid xylem surrounded by phloem (_Thamnopteris_ and _Zalesskya_); at a later stage the xylem cylinder lost its inner zone of wide and short tracheae and assumed the form seen in _Osmundites Kolbei_, in which the centre of the stele consists of parenchyma with some tracheae. Another type is represented by _O. Dowkeri_ in which the pith is composed wholly of parenchyma and the xylem ring is continuous. From this type, by expansion of the xylem ring and by the formation of overlapping leaf-gaps, the form represented by _Osmunda regalis_ was reached. _Osmunda cinnamomea_, with internal phloem in the regions of stelar branching, probably represents a further stage, as Kidston and Gwynne-Vaughan believe, in increasing complexity due to the introduction of phloem from without through gaps produced by the branching of the stele. In _Osmundites skidegatensis_ the leaf-gaps became wider and the external phloem projected deeper into the stele until a continuous internal phloem zone was produced. This most elaborate type proved less successful than the simpler forms which still survive.
_Osmundites Sturii._
Impressions of fertile pinnae with narrow linear segments bearing exannulate sporangia described by Raciborski from Lower Jurassic rocks in Poland as _Osmunda Sturii_[794] may with some hesitation be included in the list of Mesozoic Osmundaceae.
_Osmundites Dowkeri._
Under this name Carruthers[795] described a petrified stem from Lower Eocene beds at Herne Bay, which in the structure of the stele agrees closely with the Jurassic species _O. Gibbiana_ and conforms to the normal Osmundaceous type. It is possible, as Gardner and Ettingshausen[796] suggested, that the foliage of this species may be represented by some sterile _Osmunda_-like fragments recorded from the Middle Bagshot beds of Bovey Tracey and Bournemouth as _Osmunda lignitum_.
_Todites._
This generic name[797] has been applied to fossil ferns exhibiting in the structure of the sporangia and in the general habit of the fertile fronds a close resemblance to the recent species _Todea barbara_ (fig. 221, D, p. 286).
_Todites Williamsoni_ (Brongniart) figs. 256, B, C, G.
1828. _Pecopteris Williamsonis_, Brongniart, Prodrome, p. 57; Hist. vég. foss., p. 324, Pl. CX. figs. 1 and 2.
—— _P. whitbiensis_, Brongniart, Hist. vég. foss. p. 321, Pl. CIX. figs. 2–4.
—— _P. tenuis_, _ibid._ p. 322, Pl. CX. figs. 3, 4.
1829. _Pecopteris recentior_, Phillips, Geol. Yorks. p. 148, Pl. VIII. fig. 15.
—— _P. curtata_, _ibid_. Pl. VIII. fig. 12.
1833. _Neuropteris recentior_, Lindley and Hutton, Foss. Flora, Vol. I. Pl. LXVIII.
—— _Pecopteris dentata_, _ibid._ Vol. III., Pl. CLXIX.
1836. _Acrostichites Williamsonis_, Goeppert, foss. Farn. p. 285.
1841. _Neuropteris Goeppertiana_, Muenster, in Goeppert, Gattungen foss. Pflanz. Lief. 5 and 6, p. 104, Pls. VIII.–X.
1856. _Pecopteris Huttoniana_, Zigno, Flor. foss. Oolit. Vol. I. p. 133.
1867. _Acrostichites Goeppertianus_, Schenk, Foss. Flor. Grenzsch. p. 44, Pl. V. fig. 5, Pl. VII. fig. 2.
1883. _A. linnaeaefolius_, Fontaine, Older Mesoz. Flora Virginia, p. 25, Pls. VI.–IX.
— _A. rhombifolius_, _ibid_. Pls. VIII. XI.–XIV.
1885. _Todea Williamsonis_, Schenk, Palaeont. Vol. XXXI. p. 168, Pl. III. fig. 3.
1889. _Cladophlebis virginiensis_, Fontaine, Potomac Flora, p. 70, Pl. III. figs. 3–8; Pl. IV. figs. 1, 4.
[Illustration: FIG. 256.
A. _Cladophlebis denticulata._ B, B′. _Todites Williamsoni_ (fertile). C. _T. Williamsoni_ (sterile pinna). D. _Discopteris Rallii._ E, E′. _Kidstonia heracleensis._ F. _Todeopsis primaeva._ G. _Todites Williamsoni_ (sporangium).
[B, C, from specimens (13491; 39234) in the British Museum (B, very slightly reduced; C, ½ nat. size); D, E, after Zeiller; F, after Renault; G, after Raciborski.]]
It is hopeless to attempt to arrive at satisfactory conclusions in regard to the applicability of the name _Todites Williamsoni_ to the numerous fronds from Jurassic and Rhaetic rocks, agreeing more or less closely with Brongniart’s type-specimen. Specimens from the Rhaetic may not be specifically identical with those from the Jurassic; the main point is that, whether actually identical or not, both sets of fossils clearly represent the same general type of Osmundaceous fern[798] and may for present purposes be included under the same designation. The above synonymy, though by no means complete[799], serves to illustrate the confusion which has existed in regard to this widely spread type of Mesozoic fern.
_Todites Williamsoni_ may be briefly described as follows:—
Frond bipinnate; long linear pinnae (20–30 cm.) of uniform breadth arise at an acute angle, or in the lower part of a frond, almost at right angles, from a stout rachis. Closely set pinnules attached by a broad base; slightly falcate, the side towards the rachis strongly convex and the outer margin straight or concave and bulged outwards towards the base of each segment, margin usually entire, or it may be slightly lobed. Fertile pinnules similar to the sterile; sporangia of the Osmundaceous type and often scattered over the whole lower surface of the lamina (fig. 256, B, B′, G). Venation of the _Cladophlebis_ type (cf. fig. 256, A).
It is not always easy to distinguish _Todites Williamsoni_ from _Cladophlebis denticulata_, another common Jurassic fern, but in the latter the pinnules are usually longer and relatively narrower and the rachis is more slender (cf. fig. 256, B and 257). Schenk[800] and Raciborski[801] have shown that the sporangia of _Todites_ conform in the absence of a true annulus to those of _Todea_ (fig. 256, G) and _Osmunda_. Nathorst[802] has recently figured a group of spores of _Todites Williamsoni_ in illustration of the use of the treatment of carbonised impressions with nitric acid and potassium chlorate. This species, though widely distributed in Jurassic rocks, is hardly distinguishable from the German Rhaetic fronds figured by Schenk from Bayreuth as _Acrostichites Goeppertianus_[803], or from other fossils referred to an unnecessarily large number of species by Fontaine[804] from Upper Triassic rocks of Virginia[805].
It would seem from the paucity of later records of Osmundaceae that the family reached its zenith in the Jurassic era. When we pass to the later Tertiary and more recent deposits evidence is afforded in regard to the geographical range of _Osmunda regalis_. It has been shown to occur in the Pliocene forest-bed of Norfolk[806] as well as in Palaeolithic and Neolithic deposits[807].
[Illustration: FIG. 257. _Cladophlebis denticulata._ (From a specimen in the British Museum from the Inferior Oolite rocks of Yorkshire. Slightly reduced.)]
A fertile frond from the Molteno (Rhaetic) beds of South Africa referred to _Cladophlebis_ (_Todites_) _Roesserti_ (Presl)[808] represents in all probability an Osmundaceous fern closely allied to _Todites Williamsoni_. The same species is described by Zeiller[809] from Rhaetic rocks of Tonkin and very similar types are figured by Leuthardt[810] from Upper Triassic rocks of Basel as _Pecopteris Rutimeyeri_ Heer, and by Fontaine[811] from rocks of the same age in Virginia.
_Cladophlebis._
The generic name _Cladophlebis_ was instituted by Brongniart for Mesozoic fern fronds characterised by ultimate segments of linear or more or less falcate form attached to the pinnae by the whole of the base, as in the Palaeozoic genus _Pecopteris_, possessing a midrib strongly marked at the base and dividing towards the distal end of the lamina into finer branches and giving off secondary forked and arched veins at an acute angle. The term is generally restricted to Mesozoic fern fronds which, on account of the absence or imperfection of fertile pinnae, cannot be safely assigned to a particular family. In the case of the species described below, the evidence in regard to systematic position, though not conclusive, is sufficiently strong to justify its inclusion in the Osmundaceae.
_Cladophlebis denticulata_ Brongniart. Figs. 256, A; 257, 258.
1828. _Pecopteris denticulata_[812], Brongniart, Prodrome, p. 57; Hist. vég. foss. p. 301, Pl. XCVIII. figs. 1, 2.
— _P. Phillipsii_, Brongniart, Hist. p. 304, Pl. CIX. fig. 1.
This species is often confused[813] with _Todites Williamsoni_. The name _Pecopteris whitbiensis_ has been used by different writers for Jurassic fronds which are undoubtedly specifically distinct: specimens so named by Brongniart should be referred to _Todites Williamsoni_, while _P. whitbiensis_ of Lindley and Hutton[814] is Brongniart’s _Cladophlebis denticulata_. It is impossible to determine with accuracy the numerous examples described as _Pecopteris whitbiensis_, _Asplenium whitbiense_, _Cladophlebis Albertsii_ (a Wealden species[815]), _Asplenium_, or _Cladophlebis_, _nebbense_[816], etc., from Jurassic and Rhaetic strata. The _Cladophlebis denticulata_ form of frond is one of the commonest in recent ferns; it is represented by such species as _Onoclea Struthopteris_, _Pteris arguta_, _Sadleria sp._, _Gleichenia dubia_, _Alsophila lunulata_, _Cyathea dealbata_, and species of _Polypodium_. It is, therefore, not surprising to find records of this Mesozoic species from many localities and horizons. All that we can do is to point out what appear to be the most probable cases of identity among the numerous examples of fronds of this type from Mesozoic rocks,
## particularly Rhaetic and Jurassic, in different parts of the world.
The name _Cladophlebis denticulata_ may be employed in a comprehensive sense for fronds showing the following characters:—
Leaf large, bipinnate, with long spreading pinnae borne on a comparatively slender rachis. Pinnules, in nearly all cases, sterile, reaching a length of 3–4cm., acutely pointed, finely denticulate or entire, attached by the whole of the base (fig. 257). In the apical region the pinnules become shorter and broader. Venation of the _Cladophlebis_ type (fig. 256, A). Fertile pinnules rather straighter than the sterile, characterised by linear sori parallel to the lateral veins (fig. 258).
In endeavouring to distinguish specifically between fronds showing a general agreement in habit with _C. denticulata_, special attention should be paid to venation characters, the shape of the pinnules, the relation of the two edges of the lamina to one another, and to the amount of curvature of the whole pinnule. Unless the material is abundant, it is often impossible to distinguish between characters of specific value and others which are the expression of differences in age or of position on a large frond, to say nothing of the well-known variability which is amply illustrated by recent ferns. It is remarkable that very few specimens are known which throw any light on the nature of the fertile pinnae. Fig. 258 represents an impression from the Inferior Oolite rocks of the Yorkshire coast in which the exposed upper surface of the pinnules shows a series of parallel ridges following the course of the lateral veins and no doubt formed by oblong sori on the lower surface. There can be little doubt that the specimen figured by Lindley and Hutton and by others as _Pecopteris undans_[817] is, as Nathorst suggests, a portion of a fertile frond of _C. denticulata_. A fertile specimen of a frond resembling in habit _C. denticulata_, which Fontaine has described from the Jurassic rocks of Oregon as _Danaeopsis Storrsii_[818], exhibits, as that author points out, a superficial resemblance to the specimen named by Lindley and Hutton _Pecopteris undans_. There is, however, no adequate reason for referring the American fragment to the Marattiaceae. In the absence of sporangia we cannot speak confidently as to the systematic position of this common type; but there are fairly good grounds for the assertion that some at least of the fronds described under this name are those of Osmundaceae. The English specimen shown in fig. 258 is very similar to some Indian fossils figured by Feistmantel as _Asplenites macrocarpus_[819], which are probably identical with _Pecopteris australis_ Morris[820], a fern that is indistinguishable from _Cladophlebis denticulata_. Renault[821] figured a fertile specimen of the Australian fossil as _Todea australis_, which agrees very closely with that shown in fig. 258, and the sporangia figured by the French author are of the Osmundaceous type. Another example of a fertile specimen is afforded by a Rhaetic fern from Franconia, _Asplenites ottonis_, which is probably identical with _Alethopteris Roesserti_ Presl [= _Cladophlebis_ (_Todites_) _Roesserti_], a plant closely resembling _Cladophlebis denticulata_. Another argument in favour of including _C. denticulata_ in the Osmundaceae is supplied by the association of pinnae of this type with the petrified stem of _Osmundites Dunlopi_ recorded by Kidston and Gwynne-Vaughan.
[Illustration: FIG. 258. Fertile pinnae of _Cladophlebis denticulata_. (From a Yorkshire specimen in the Sedgwick Museum, Cambridge.)]
=Schizaeaceae.=
Evidence bearing on the existence of this family in Carboniferous floras is by no means decisive. The generic name _Aneimites_ proposed by Dawson[822] for some Devonian Canadian plants resembling species of the recent genus _Aneimia_, and adopted by White[823] for a species from the Pottsville beds of Virginia, is misleading. The Canadian plants give no indication of the nature of the reproductive organs, and the fronds described by White are, as he shows, those of a Pteridosperm and bore seeds.
An examination of the suspiciously diagrammatic drawings published by Corda[824] of the small fertile pinnules of a Carboniferous fern from Bohemia, which he named _Senftenbergia elegans_, leads us to conclude that the sporangia are almost certainly those of a Schizaeaceous species. The small linear pinnules bear two rows of sessile sporangia, singly as in recent Schizaeaceae and not in sori, characterised by 4–5 rows of regular annular cells (fig. 270, A) surrounding the apex. It has already been pointed out that the apical annulus of recent Schizaeaceae, though normally one row deep, may consist in part at least of two rows. Zeiller[825] examined specimens of Corda’s species and decided in favour of a Schizaeaceous affinity; he describes the sporangia as 0·85–0·95 mm. in length, with 3 to 5 and occasionally only two rows of cells in the apical annulus. Zeiller’s figures (fig. 270, A) confirm the impression that Corda’s drawings are more beautiful than accurate. Stur[826], on the other hand, who first pointed out that the type-specimens of _Senftenbergia_ came from the Radnitz beds of Bohemia and not from the Coal-Measures, convinced himself that the sporangia have no true annulus (fig. 270, E). He describes them as characterised by a comparatively strong wall and by the presence of a band of narrow vertical cells marking the line of dehiscence, features which lead him to assign the plant to the Marattiales, a group which seems to have exercised a dominating influence over his judgment. In a later publication Zeiller[827] replies to Stur’s criticism but adheres to his original opinion. Solms-Laubach[828], while expressing himself in favour of Marattiaceous affinity, recognises that Zeiller’s arguments cannot be set aside.
The question must remain open until further evidence is forthcoming; but it would seem that this Carboniferous type, not as yet recognised in Britain, possessed sporangia having a distinct resemblance to those of the Schizaeaceae, though this similarity does not amount to proof of the existence of the family in the Palaeozoic era.
Palaeozoic floras may be described as rich in generalised types, types foreshadowing lines of evolution, which in the course of ages led to a sorting and a redistribution of characters. It may be that _Senftenbergia_ is one of these generalised types.
• • • • •
It is not until we ascend the geological series as far as the older Jurassic rocks that we meet with a type which can with confidence be classed with the Schizaeaceae, as least so far as sporangial characters are concerned. The species _Klukia exilis_ is selected as the best known and most widely-spread representative of Jurassic Schizaeaceae.
_Klukia exilis_ (Phillips)[829]. Fig. 259.
The generic name _Klukia_ was proposed by Raciborski[830] for a species originally described by Phillips[831] from the Inferior Oolite of the Yorkshire coast as _Pecopteris exilis_. Bunbury’s[832] discovery (supplemented by additional evidence obtained by Raciborski) of well-preserved sporangia justified the substitution of a distinctive designation for the provisional term _Pecopteris_.
[Illustration: FIG. 259. _Klukia exilis_ (Phillips). (Figs. 1–3, × 40; fig. 4, × 3; fig. 5, nat. size.)]
The species may be defined as follows:—
Frond tripinnate, of the _Cladophlebis_ type; pinnae linear, lanceolate, attached to the rachis at a wide angle. Ultimate segments short and linear, entire or, in the lower part of a frond, crenulate, 5 mm. long or occasionally longer. Sporangia 0·5 mm. in length, borne singly on the lower surface of the lamina in a row on each side of the midrib.
A re-examination[833] of the specimen described by Bunbury confirmed his account of the structure of the sporangia. The pinna shown in fig. 259 is characterised by unusually small fertile pinnules some of which bear 10 sporangia in two rows; the annulus includes about 14 cells. Fertile specimens of this and similar forms are figured by Raciborski[834] from Jurassic rocks of Poland, and good examples of the English species may be seen in the Leckenby collection, Cambridge, in the British Museum, the museums of Manchester, Scarborough, and other places.
It is possible that specimens referred to _K. exilis_ by Yokoyama[835] from Wealden strata in Japan may afford evidence of the persistence of the species beyond the Jurassic era, but in view of the close resemblance of the sterile fronds described from Wealden strata as _Cladophlebis Brownii_[836] and _C. Dunkeri_[836] to those of _Klukia exilis_, identity can be established only by an examination of fertile specimens. A Jurassic fern recently described by Yabe[837] from Korea as _Cladophlebis koraiensis_ may be identical with _K. exilis_ and there is little doubt as to the existence of the species in Jurassic Caucasian strata[838].
[Illustration: FIG. 260. _Ruffordia Goepperti_. (A, C, sterile; B, fertile; slightly reduced. Specimens from the Wealden of Sussex; British Museum; V. 2333, V. 2160, V. 2166.)]
_Ruffordia Goepperti_ (Dunk.). Fig. 260.
This Wealden fern[839] has been doubtfully assigned to the Schizaeaceae on the ground of the resemblance of the sterile fronds to those of some species of _Aneimia_, and because of the difference between the sterile and fertile pinnae (Fig. 260). _Ruffordia_ cannot be regarded as a well authenticated member of the Schizaeaceae.
[Illustration: FIG. 261.
A, A′. _Chrysodium lanzaeanum._ B, B′. _Lygodium Kaulfussi._ C. _Marattia Hookeri._
(After Gardner and Ettingshausen; A, B, ¾ nat. size.)]
_Lygodium Kaulfussi_, Heer. Fig. 261, B, B′.
Fragments of forked pinnules, agreeing very closely in venation and general appearance with recent species of _Lygodium_, have been identified by Gardner and Ettingshausen[840] from English Eocene beds and by Knowlton from the Miocene beds of the Yellowstone Park[841] as _Lygodium Kaulfussi_ Heer (fig. 261, B). Despite the absence of sporangia it is probable that these fragments are correctly referred to the Schizaeaceae. The sterile and fertile specimens figured by Heer[842] from Tertiary beds of Switzerland agree very closely with recent examples of _Lygodium_. Similar though perhaps less convincing evidence of the existence of this family in Europe is furnished by Saporta[843], who described two Eocene species from France.
=Gleicheniaceae.=
The application by Goeppert[844] and other earlier writers of the generic name _Gleichenites_ to examples of Palaeozoic ferns was not justified by any satisfactory evidence. One of Goeppert’s species, _Gleichenites neuropteroides_, is identical with _Neuropteris heterophylla_[845], a plant now included in the Pteridosperms.
The resemblance of sporangia and sori, whether preserved as carbonised impressions or as petrified material, from Carboniferous rocks, to those of recent species of Gleicheniaceae is in many cases at least the result of misinterpretation of deceptive appearances. Williamson[846] drew attention to the Gleichenia-like structure of some sections of sporangia from the English Coal-Measures, but he did not realise the ease with which sections of Marattiaceous sporangia in different planes may be mistaken for those of annulate (leptosporangiate) sporangia. In the regular dichotomous habit of Carboniferous fronds described as species of _Diplothmema_ (Stur) and _Mariopteris_ (Zeiller)[847] we have a close correspondence with the leaves of _Gleichenia_, but the common occurrence of dichotomous branching among ferns is sufficient reason for regarding this feature as an untrustworthy criterion of relationship. It is, however, interesting to find that in addition to the existence of some Upper Carboniferous ferns with sori like those of recent Gleichenias, the type of stelar anatomy illustrated by _Gleichenia dicarpa_ (fig. 237, C, p. 310) and other species is characteristic of the primary structure of the stem of the Pteridosperm _Heterangium_. We find in Carboniferous types undoubted indications of anatomical and other features which in succeeding ages became the marks of Gleicheniaceae.
Some Carboniferous fronds with short and small pinnules of the _Pecopteris_ type, bearing sori composed of a small number of sporangia, have been assigned by Grand’Eury and other authors to the Gleicheniaceae; the same form of sorus is met with also on fronds with Sphenopteroid segments. The former is illustrated by _Oligocarpia Gutbieri_[848] and the latter by _O. Brongniarti_ described by Stur and by Zeiller[849]. Zeiller has described the circular sori of _Oligocarpia_ (fig. 270, B) as consisting of three to ten pyriform sporangia borne at the ends of lateral veins and possessing a complete transverse annulus, but Stur[850] believes that the annulus-like appearance is due to the manner of preservation of exannulate sporangia. In this opinion Stur is supported by Solms-Laubach[851] and by Schenk[852]. Despite an agreement between _Oligocarpia_ and _Gleichenia_, as regards the form of the sori and the number of sporangia, it is not certain that the existence of a typical Gleicheniaceous annulus has been proved to occur in any Palaeozoic sporangia[853].
From Upper Triassic beds of Virginia, Fontaine has figured several fronds for which he instituted the genus _Mertensides_[854]. The habit, as he points out, is not dichotomous, but the sori are circular and are said to be composed in some species of four to six sporangia. No satisfactory evidence is brought forward in support of the use of a designation implying a close relationship with recent Gleichenias (sect. _Mertensia_). One of the species described by Fontaine was originally named by Bunbury _Pecopteris bullatus_[855], the imperfect type-specimen of which is now in the Museum of the Cambridge Botany School. In the form of the frond, the thick rachis, and in the pinnules this Triassic species resembles _Todites Williamsoni_, but the resemblance does not extend to the sori. Two of Fontaine’s species are recorded by Stur from Austria[856], but he places them in the genus _Oligocarpia_ and includes them in the Marattiaceae.
Leuthardt[857] figures what appears to be a Gleicheniaceous fern from the Upper Triassic beds of Basel as _Gleichenites gracilis_ (Heer) showing sori composed of five sporangia (fig. 265, C) with a horizontal annulus. A Rhaetic species _Gleichenites microphyllus_ Schenk[858] from Franconia agrees in the form of its small rounded pinnules with _Gleichenia_, but no sporangia have so far been found.
An impression of a frond from Jurassic rocks of northern Italy figured by Zigno as _Gleichenites elegans_[859] closely resembles in habit recent species of _Gleichenia_; though no sporangia have been found, the habit of the frond gives probability to Zigno’s determination.
A Jurassic species from Poland, _Gleichenites Rostafinskii_, referred by Raciborski[860] to _Gleichenia_, exhibits a close agreement in habit and in the form of the soral impressions to some recent species of _Gleichenia_.
As we pass upwards to Wealden and more recent rocks it becomes clear that the Gleicheniaceae were prominent members of late Mesozoic floras in north Europe and reached as far north as Disco Island. In English Wealden beds portions of sterile fronds have been found which were assigned to a new genus _Leckenbya_[861], but it is probable that these specimens would be more correctly referred to _Gleichenites_. Similarly fragments of Gleichenia-like pinnae with very small rounded pinnules occur in the Wealden rocks of Bernissart, Belgium[862], in north Germany[863], and elsewhere. Conclusive evidence has been obtained by Prof. Bommer of the existence of _Gleichenites_ in Wealden beds near Brussels, where many plant remains have been found in a wonderful state of preservation. The specimens, which I had an opportunity of seeing some years ago, might easily be mistaken for rather old and brown pieces of recent plants. Some of the Belgian fragments, of which Prof. Bommer has kindly sent me drawings and photographs, are characterised by an arrangement of vascular tissue identical with that in the petioles and rhizomes of some protostelic Gleichenias. The stele of one of the Belgian rhizomes appears to be identical with that of _Gleichenia dicarpa_ (fig. 237, C. p. 310).
[Illustration: FIG. 262.
A. _Gleichenites longipennis_ Heer. B. _G. delicatula_ Heer. C. _G. Nordenskioldi_ Heer. D. _G. Zippei_. (Corda.)
(After Heer; A, B, D, very slightly reduced.)]
_Gleichenites Zippei_ (Corda). Fig. 262, D.
This species, originally described by Corda as _Pecopteris Zippei_[864] and afterwards figured by Heer[865] as _Gleichenia Zippei_ (fig. 262, D) from Urgonian rocks of Greenland, affords a striking example of a Mesozoic member of the _Gleicheniaceae_. It is characterised by the dichotomous branching of the frond and by the occurrence of arrested buds in the forks. The long and slender pinnae, reaching a length of 9 cm. and a breadth of 6–8 mm., bear small crowded pinnules occasionally with circular sori which are described by Heer as consisting of a small number of sporangia (cf. fig. 262, C). Several other Lower Cretaceous species are recorded by Heer from Greenland, some of which are probably unnecessarily separated from _Gleichenites Zippei_. Examples of these are represented in fig. 262, A, B, C.
A Gleicheniaceous species described by Debey and Ettingshausen from Lower Cretaceous rocks of Aix-la-Chapelle as _Didymosorus comptonifolius_[866] is very similar in habit to some of Heer’s Greenland species: this should probably be referred to the genus _Gleichenites_.
_Gleichenites hantonensis_, Wank. Fig. 263.
From the Eocene beds of Bournemouth, Gardner and Ettingshausen[867] have described under the name _Gleichenia hantonensis_ what is in all probability a true _Gleichenia_ (fig. 263). This species, originally recorded by Wanklyn[868], is characterised by a slender forked rachis showing what may be traces of arrested buds between the arms of the branches, by circular sori of six or eight sporangia and by the presence of peculiar tendril-like appendages on the pinnae. If the description of the tendrils is correct, this British species affords one of the few instances of ferns adapted for climbing and may be compared with the recent species _Davallia aculeata_ (fig. 232, p. 299).
=Matonineae.=
The genera _Laccopteris_ and _Matonidium_ may be described as examples of Mesozoic ferns exhibiting a very close agreement with _Matonia_.
_Laccopteris_. This genus, founded by Presl[869], may be described as follows:—
Frond pedate, in habit resembling _Matonia pectinata_, with pinnate or pinnatifid pinnae; ultimate segments linear, provided with a well-marked midrib giving off numerous dichotomously branched secondary veins which are in places connected by lateral anastomoses. Sori circular, forming a single row on each side of the midrib (fig. 278, B); sporangia 5–15 in each sorus, with an oblique annulus and tetrahedral spores. The presence of an indusium is not certainly established.
[Illustration: FIG. 263. _Gleichenites hantonensis_ Wank. (Restoration, after Gardner and Ettingshausen.)]
Schenk[870], who described several specimens of _Laccopteris_ from Rhaetic rocks of Germany, compared the genus with _Gleichenia_ but he also recognised the close resemblance to _Matonia pectinata_. Zeiller[871] first established the practical identity of the sori and sporangia of _Laccopteris_ and _Matonia_. The Rhaetic species, such as _L. Muensteri_, _L. elegans_, and _L. Goepperti_, agree very closely with _L. polypodioides_ and need not be described in detail.
[Illustration: FIG. 264. _Laccopteris elegans_ (Presl). (From a specimen in the British Museum; from the Lower Keuper of Bayreuth, Germany. Nat. size; part of pinnule × 3.)]
The Rhaetic species _Laccopteris elegans_, represented in fig. 264, illustrates the characteristic habit of the genus and shows a feature usually overlooked[872], namely the occurrence of anastomoses between the lateral veins. The form of the sorus of another Rhaetic species is shown in fig. 265, E. Schenk figures an interesting series of fronds of _L. Goepperti_ in different stages of growth[873]; one of the younger leaves is seen in fig. 265, D. An examination of Rhaetic specimens of _Laccopteris_ in the Bergakademie of Berlin convinced me of the correctness of the published descriptions of the sori.
[Illustration: FIG. 265.
A. _Matonidium Wiesneri._ (Slightly enlarged.) B. _Marattiopsis marantacea._ (Slightly enlarged.) C. _Gleichenites gracilis._ (Slightly enlarged.) D. _Laccopteris Goepperti._ (Slightly reduced.) E. _L. Muensteri._ (Enlarged.)
(A, after Krasser; B, C, after Leuthardt; D, E, after Schenk.)]
_Laccopteris polypodioides_ (Brongniart). Figs. 266–268; 278, A.
1828. _Phlebopteris polypodioides_[874], Brongniart, Hist. vég. foss. p. 372, Pl. LXXXIII. fig. 1.
— _P. propinqua_, _ibid._ Pls. CXXXII. fig. 1, CXXXIII. fig. 2.
1829. _Pecopteris caespitosa_, Phillips, Geol. Yorks. p. 148, Pl. VIII. fig. 10.
— _P. crenifolia_, _ibid._ Pl. VIII. fig. 10.
— _P. ligata_, _ibid._ Pl. VIII. fig. 14.
[Illustration: FIG. 266. _Laccopteris polypodioides_ (Brongn.). (× 14.) (Brit. Mus.)]
In habit this species closely resembles _Matonia_ and _Matonidium_, the long petiole divides distally into several spreading pinnatifid pinnae with linear ultimate segments (fig. 278, A). Circular sori (indusiate?) occur in a single row on each side of the midrib containing 12–14 large sporangia (fig. 266) characterised by an obliquely vertical annulus. The midrib of the pinnules gives off secondary veins at a wide angle and these form a series of elongated meshes parallel to the median rib, as in the recent genus _Woodwardia_; forked and anastomosing branches are given off from these to the edge of the lamina (fig. 267).
[Illustration: FIG. 267. Pinnules of _Laccopteris_. (Enlarged.)
A, B. From the Inferior Oolite of Yorkshire. C. From the Inferior Oolite of Stamford. (British Museum.)]
The specimen shown in fig. 268 is probably a young frond of this species.
A very similar, possibly a specifically identical plant, was described by Leckenby from English Jurassic rocks as _Phlebopteris Woodwardi_[875], the distinguishing features of which are the greater number of lateral veins and the smaller sori (fig. 267, A).
The name _Microdictyon_ was proposed by Saporta[876] for pinnules differing slightly from those of _Laccopteris_ in venation characters: he included _Laccopteris Woodwardi_ in this genus, but such differences as are recognisable in the venation hardly justify the use of a distinct generic title. Similarly, specimens described by Debey and Ettingshausen[877] from Lower Cretaceous rocks of Aix-la-Chapelle as species of _Carolopteris_ may also be included in _Laccopteris_.
[Illustration: FIG. 268. ? _Laccopteris polypodioides._ Nat. size. From a specimen in the Whitby Museum (Brit. Mus.).]
_Laccopteris Dunkeri_ (Schenk)[878].
This species is represented in several Wealden localities by fragments of fertile pinnae similar to those of _L. polypodioides_. It is almost impossible to distinguish small specimens of the Wealden fern from Heer’s genus _Nathorstia_ (Marattiaceae) unless the sori are well preserved. This species occurs in Wealden beds in England, Germany, Belgium, and elsewhere and has been discovered by Dr Marcus Gunn in Upper Jurassic plant-beds of Sutherlandshire (N.E. Scotland).
• • • • •
_Laccopteris_ is widely spread in Rhaetic, Jurassic and Lower Cretaceous floras. It affords evidence of the former abundance in northern latitudes of a family now represented by the two species of _Matonia_ confined to a restricted area in the southern hemisphere.
_Matonidium._
Schenk[879] instituted this convenient term for fossil fern fronds agreeing in habit and in their sori with _Matonia pectinata_ (figs. 227, 228, p. 292). Zeiller[880] has drawn attention to the fact that the Mesozoic species differ from the surviving types in the greater number of sporangia in each sorus, and, it may be added, in _Matonidium_ the fertile pinnules are more richly supplied with sori than are those of _Matonia_. Unfortunately our knowledge of the structure of the sporangia of _Matonidium_ is less complete than in the case of _Laccopteris_, but such evidence as is available justifies the conclusion that _Matonia_ is a direct descendant of ferns which formed a prominent feature in European Jurassic and Wealden floras. It is interesting to find that in a Cretaceous species, described by Krasser (fig. 265, A) since the publication of Zeiller’s paper, the sori appear to be identical in distribution and in appearance with those of the recent species.
I am indebted to Prof. Bommer for permission to reproduce the unpublished drawing represented in fig. 237 D (p. 310) of a section of the rhizome of _Matonidium_ from the Belgian Wealden beds of Hainaut (“Flore Bernissartienne”). The section shows an arrangement of vascular tissue identical with that in the recent species: there may be two solenosteles and in addition a solid axial strand. The form of the leaf-trace in the fossil appears to be identical with that in _Matonia pectinata_ (fig. 237, A, p. 310).
_Matonidium Goepperti_ (Ettingshausen)[881]. Fig. 269.
Under this name are included specimens from Inferior Oolite and Wealden strata in Britain and elsewhere. It is, however, not impossible that if more information were available, we should find adequate reasons for recognising two specific types. Fontaine[882], adhering rigidly to the rules of priority, speaks of this species as _Matonidium Althausii_ (Dunker), but Ettingshausen’s specific term is better known.
[Illustration: FIG. 269. _Matonidium Goepperti_ (Ettings.). (A, B, ½ nat. size; C, approximately nat. size.)]
Fronds pedate and apparently identical in habit with those of _Matonia pectinata_; ultimate segments linear, slightly falcate and bluntly pointed. Sori circular or oval, numerous, containing 15 to 20 sporangia with an oblique annulus, in two rows on the lower surface of the pinnules; indusium as in _Matonia_.
The English examples have so far afforded no information in regard to sporangial structure, but Schenk[883] has recognised a distinct annulus in German material. In his description of fossil plants from Lower Cretaceous rocks in California, Fontaine[884] doubtfully identifies two very small fragments as _Matonidium Althausii_; the evidence is, however, wholly inadequate.
_Matonidium Wiesneri_, Krasser[885]. Fig. 265, A.
This Cenomanian (Cretaceous) species from Moravia appears to be identical in habit with the older type. The pinnules are larger and bear fewer sori. Krasser’s figures of the sterile pinnules show no lateral anastomosing between the secondary veins, but the small vascular network below each sorus (fig. 265, A) is identical with that in _Matonia pectinata_. The indusiate sori contain about six sporangia with an oblique annulus.
The very wide geographical distribution of the Matonineae during the Mesozoic era affords a striking contrast to the limited range of the Malayan survivals.
=Hymenophyllaceae.=
The frequent use of the generic name _Hymenophyllites_ as a designation of Palaeozoic ferns, more particularly in the older literature, is another instance of the undue importance which palaeobotanists have always been prone to attach to external resemblances of vegetative organs. The fragment of lamina described by Stur for the Culm Measures of Austria as _Hymenophyllum waldenburgense_[886] has no claim to consideration as evidence of Palaeozoic Hymenophyllaceae. On the other hand, there are a few records of fertile fronds which, though not to be accepted without reserve, are worthy of more careful examination. Some petrified sporangia described by Renault[887] from the Culm of Esnost are referred to _Hymenophyllites_ on account of the position of the annulus, which appears to encircle about two-thirds of the circumference; it is, however, not certain that the annulus is horizontal as in the recent genus.
The Culm species _Rhodea patentissima_ described by Ettingshausen[888] as _Hymenophyllites patentissima_ and subsequently referred by Stur[889] to _Rhodea_, is regarded by these authors as closely allied to _Hymenophyllum_ simply on the ground of the finely divided and delicate sterile fronds; another species, _Rhodea moravica_ (Ett.), which Ettingshausen referred to _Trichomanes_, is compared with recent species of that genus. In neither case do we know anything of sporangial characters.
[Illustration: FIG. 270.
A, E. _Senftenbergia elegans._ B. _Oligocarpia Brongniartii._ C. _Trichomanes_ sp. D. _Hymenophyllum tunbrigense._ F, G. _Sphenopteris_ (_Hymenophyllites_) _quadridactylites._
(A, B, F, G, after Zeiller; D, after Hooker; E, after Stur.)]
A fertile sphenopteroid frond figured by Schimper as _Hymenophyllum Weissi_[890] from the Coal-Measures of Saarbrücken bears some resemblance to recent Hymenophyllaceae, but the figures are by no means convincing: an examination of the type-specimens in the Strassburg Museum led Solms-Laubach[891] to express dissent from Schimper’s determination. A more satisfactory example is that afforded by the fertile pieces of a frond described by Zeiller[892] from French Coal-Measures as _Hymenophyllites quadridactylites_ (Gutbier). Some of the ultimate segments with a truncated tip are preserved in close association with a group of oval sporangia with a complete transverse annulus (fig. 270, F, G). The position of the sporangia is such as to suggest their separation from a terminal columnar receptacle like that in _Trichomanes_ and _Hymenophyllum_. In his account of this species from the Coal-Measures of the Forest of Wyre, Kidston[893] states that Zeiller informed him that he had noticed traces of what appeared to be a columnar receptacle in the French specimens.
The records of Hymenophyllaceae from the Mesozoic and Tertiary formations are not such as need detain us. The facts bearing on the geological history of this family are singularly meagre. There is no evidence which can be adduced in favour of regarding the Hymenophyllaceae as ferns of great antiquity, which played a prominent
## part in the floras of the past.
It is interesting to find that the genus _Ankyropteris_[894], one of the Botryopterideae (a group of Palaeozoic Ferns for which I propose the name Coenopterideae), has a morphological character in common with _Trichomanes_, namely the production of axillary buds: there are also features in the stelar anatomy shared by the Botryopterideae and Hymenophyllaceae[895]. These resemblances, though by no means amounting to proof of near relationship, point to a remote ancestry for certain features retained by existing members of the Hymenophyllaceae.
=Cyatheaceae.=
The specimens from the Culm rocks of Moravia on which Stur founded the species _Thyrsopteris schistorum_[896] are too imperfectly preserved to warrant the use of this generic name. Goeppert[897] in 1836 instituted the genera _Cyatheites_, _Hemitelites_, and _Balantites_ for species of Carboniferous ferns believed to be closely allied to recent Cyatheaceae, but a fuller knowledge of these types has clearly demonstrated that in all cases the reference to this family had no justification.
The Upper Carboniferous species _Dicksonites Pluckeneti_, of which Sterzel[898] described fertile specimens in 1886 as possessing circular sori, has since been shown by Grand’Eury[899] to be a Pteridosperm bearing small seeds. In _Sphenopteris_ (_Discopteris_) _cristata_ (Brongn.) Zeiller[900] has described sori very like those of _Cyathea_ and _Alsophila_, but differing in the exannulate sporangia: this species, like so many of the Palaeozoic ferns, is probably more akin to the Marattiaceae than to the Cyatheaceae.
We have as yet no satisfactory evidence of the existence of the Cyatheaceae in Palaeozoic floras. It is not until we reach the Jurassic period that trustworthy data are obtained. Raciborski[901] has identified as Cyatheaceous fertile Jurassic fronds from Poland, but his figures are inconclusive. In _Alsophila polonica_ it is not clear whether the annulus is vertical or oblique, and in another supposed member of the family, _Gonatosorus Nathorsti_, in which the indusium is described as bivalvate, there is no proof of affinity to Cyatheaceae.
In attempting to decipher the past history of the Cyatheaceae it is important to remember the close resemblance between the fertile segments of some species of _Davallia_ (Polypodiaceae) and those of _Dicksonia_ (fig. 229, C, D, p. 294). Unless the sporangia are well enough preserved to show the position of the annulus, it is frequently impossible to feel much confidence in the value of the grosser features, such as the reduced lamina of the fertile segments and the form of the sori. It is, however, probable that the widely-spread Jurassic species _Coniopteris hymenophylloides_ is correctly referred to the Cyatheaceae, but even in the case of this species the evidence of external form needs confirmation by an examination of individual sporangia.
_Coniopteris._
This genus was instituted by Brongniart[902] for fossil fronds characterised by pinnules more or less intermediate between the _Pecopteris_ and _Sphenopteris_ type and agreeing in the form of the sori with the leaves of recent species of _Dicksonia_. It should be noted that Stur included in this genus a species, _Coniopteris lunzensis_[903] from the Upper Trias of Lunz, which he regarded as a Marattiaceous fern.
_Coniopteris hymenophylloides_, Brongn. Figs. 271, 272, 275, B.
1828. _Sphenopteris hymenophylloides_, Brongniart, Hist. vég. foss. p. 189, Pl. LVI. fig. 4.
1829. _S. stipata_, Phillips, Geol. York. p. 147, Pl. X. fig. 8.
1835. _Tympanophora simplex_, Lindley and Hutton, Foss. Flor. Pl. CLXX. A.
— _T. racemosa_, _ibid._ Pl. CLXX. B.
— _Sphenopteris arguta_, _ibid._ Pl. CLXVIII.
1836. _Hymenophyllites Phillipsi_, Goeppert, Foss. Farn. p. 256.
1849. _Coniopteris hymenophylloides_, Brongniart, Tableau, p. 105.
— _Coniopteris Murrayana_, _ibid._
1851. _Sphenopteris nephrocarpa_, Bunbury, Quart. Journ. Geol. Soc. Vol. VII. p. 129, Pl. XII. fig. 1.
1876. _Thyrsopteris Murrayana_, Heer, Flor. Foss. Arct. Vol. IV. (2) p. 30, Pls. I. II. VIII.
The above list represents a small selection of the names applied to Jurassic ferns from different localities which there are good grounds for regarding as referable to a single type[904].
Frond tripinnate; pinnae linear acuminate, attached to the rachis at a wide angle; the pinnules vary considerably in size and shape; in some the lamina is divided into a few broad and rounded lobes (fig. 275, B) while in others the leaflets are dissected into narrow linear segments. The sori are borne at the ends of veins; the fertile pinnules have a much reduced lamina and, in extreme cases, bear a close resemblance to those of _Thyrsopteris elegans_ (fig. 229, A, p. 294). The sori are partially enclosed in a cup-like indusium and the sporangia appear to have an oblique annulus.
Venation and habit of frond of the _Sphenopteris_ type.
[Illustration: FIG. 271. _Coniopteris hymenophylloides_ (Brongn.). Nat. size. From a specimen in the Manchester Museum.]
The pinna shown in fig. 271 is the type-specimen of _Sphenopteris arguta_ Lind. and Hutt. from the Yorkshire Inferior Oolite and is indistinguishable from the English examples on which Brongniart founded his species _S. hymenophylloides_. Fig. 272 shows a specimen from the York Museum illustrating the difference between the sterile and fertile pinnae. The resemblance of some fertile pinnae of _Coniopteris hymenophylloides_ to those of _Thyrsopteris elegans_ has led to a frequent use, without any solid justification, of the generic name of the Juan Fernandez fern for Jurassic and Wealden plants. It is not impossible that some of the fossils described by Heer from Jurassic rocks of Siberia[905] as species of _Thyrsopteris_ are Cyatheaceous ferns, but it is impossible to say with certainty that they are generically identical with the recent species. In his monograph of the Potomac flora of Virginia[906] and Maryland, Fontaine has described as species of _Thyrsopteris_ several specimens of fronds which afford no evidence as to the nature of the sori or sporangia. Some of the fronds referred by this author to _Thyrsopteris rarinervis_[907], which I examined in the Washington Museum, are in all probability examples of _Onychiopsis_, a genus included in the Polypodiaceae. The fragments described by Lester Ward[908] as species of _Thyrsopteris_ from the Lower Cretaceous of the Black Hills of North America afford no satisfactory evidence of relationship to the recent type. Similarly Velenovský has described a Lower Cretaceous _Onychiopsis_ from Bohemia[909] as a species of _Thyrsopteris_, although the fertile segments bear little or no resemblance to those of the Cyatheaceous genus. Some fertile portions of fronds described by Heer[910] as _Asplenium Johnstrupi_ and afterwards as _Dicksonia Johnstrupi_[911] from the Cretaceous beds (Kome series) of Greenland are very similar to _Coniopteris hymenophylloides_.
[Illustration: FIG. 272. _Coniopteris hymenophylloides._ Specimen from the Inferior Oolite, Scarborough; in the York Museum. [M.S.]]
_Coniopteris quinqueloba_ (Phillips). Fig. 273.
This species, originally described by Phillips[912] as _Sphenopteris quinqueloba_, is very similar in habit to _C. hymenophylloides_, differing chiefly in the smaller size of the leaf and in the narrower ultimate segments. The specimen shown in fig. 273, B, illustrates the form of the sorus and sporangia.
[Illustration: FIG. 273. _Coniopteris quinqueloba_ (Phillips). A, × 2; B, considerably enlarged. From drawings supplied by Dr Nathorst.]
_Coniopteris arguta_ (Lind. and Hutt.[913]). Figs. 274, 275, A.
The sterile pinnae of this species bear pinnules of a type met with in various species of ferns from different horizons; the smaller ones are entire and slightly falcate, while on the lower part of a frond the ultimate segments are longer and have a crenulate margin. The fertile pinnae bear pinnules reduced to a midrib with a narrow border, and terminating in a cup-like indusium (fig. 275, A). In habit the sterile leaf (fig. 274) of this species is similar to the Jurassic Schizaeaceous fern _Klukia exilis_.
_Protopteris._
Presl[914] instituted this genus for a Lower Cretaceous tree-fern from Bohemia originally figured as _Lepidodendron punctatum_[915] and assigned to a Palaeozoic horizon; it was afterwards named by Corda[916] _Protopteris Sternbergii_ and referred by Brongniart[917] to _Sigillaria_. The genus _Protopteris_ stands for fossil fern-stems with the habit and, in the main, the structural features of recent tree-ferns. Persistent leaf-bases and sinuous adventitious roots cover the surface of the stems: the vascular system is of the dictyostelic type characteristic of _Cyathea_ (fig. 240, p. 313) and _Alsophila_. It is by the pattern formed by the vascular tissue on the exposed surface of the leaf-bases that _Protopteris_ is most readily recognised: the leaf-trace has a horse-shoe form with the ends curled inwards and the sides more or less indented (fig. 277). The generic name _Caulopteris_ is used by some authors in preference to Presl’s genus; but _Protopteris_ is more conveniently restricted to Mesozoic Cyatheaceous stems and _Caulopteris_ to Palaeozoic stems, with the internal structure of _Psaronius_ (see Chap. XXIII.). Stenzel applies _Caulopteris_ to Mesozoic stems in which the leaf-trace consists of several separate strands and not of a continuous band.
[Illustration: FIG. 274. _Coniopteris arguta._ (Nat. size. From a specimen in the Sedgwick Museum, Cambridge.)]
[Illustration: FIG. 275.
A. _Coniopteris arguta._ (Fertile pinnae; nat. size.) B. _C. hymenophylloides._
A, from the Inferior Oolite of Yorkshire (British Museum); B, from Jurassic rocks in Turkestan. ]
Lower Cretaceous casts of tree-fern stems in the Prague Museum have been described under the names _Alsophilina_ and _Oncopteris_; the figures of the latter (fig. 276) given by Feistmantel[918] and by Velenovský[919] show the petiole-bases arranged in vertical rows and characterised by leaf-traces consisting of two separate strands in the form of two =V=s lying on their sides.
Tree-fern stems described under various generic names are not infrequently found in European Lower Cretaceous rocks: their comparative abundance affords an example of striking changes in geographical distribution since the latter part of the Mesozoic epoch. The Cyatheaceae no longer exist in Europe and the arborescent species of the genus have retreated to more southern regions.
[Illustration: FIG. 276. _Oncopteris Nettvalli._ (After Velenovský; ¾ nat. size.)]
[Illustration: FIG. 277. _Protopteris punctata._ (After Heer; very slightly reduced.)]
_Protopteris punctata_ (Sternb.). Fig. 277.
The earliest information in regard to the anatomy of this widely spread Lower Cretaceous fern we owe to Corda, who showed that the species agrees in essentials with existing tree-ferns. The English example described by Carruthers[920] from Upper Greensand beds in Dorsetshire (now in the British Museum) shows only the external features. The sandstone cast (14 cm. in diameter), of which a portion is seen in fig. 277, was described by Heer from Disco Island (Greenland) as a Carboniferous species[921], but afterwards correctly assigned to the Cenomanian series[922] This species is recorded also from the Lower Cretaceous of Bohemia by Frič and Bayer[923] Among examples of petrified stems exhibiting a general agreement with _Protopteris punctata_ are those described by Stenzel[924] from Turonian rocks in Germany. In one of these, _Rhizodendron oppoliense_ Göpp., attention is drawn to branches given off from the stem stele which have a solenostelic structure in contrast to the dictyostele of the stem; also to the minute structure of the tracheae which appear to have their ends perforated, a feature shown by Gwynne-Vaughan[925] to be characteristic of the xylem elements of many ferns.
[Illustration: FIG. 278.
A. _Laccopteris polypodioides_, Brongn. [From a specimen (39275) in the British Museum; slightly reduced.] B. _L. Muensteri._ C. _Dicksonia_ (petiole stele). D. _Onychiopsis Mantelli_ (fertile segments). E. _Hausmannia Sewardi_ Richt. F. _H. Kohlmanni_ Bicht. G, H. _Protopteris Witteana_, Schenk. (x, xylem; R, roots.)
(B, after Schenk; E, F, after Richter.)]
_Protopteris Witteana_ Schenk[926] (fig. 278, G, H), a Wealden species recorded from Germany and England, represents a closely allied or possibly an identical type. The section of the stem (fig. H) shows the narrow vascular bands, x, of a dictyostele similar to that of recent Cyatheaceous tree-ferns and a form of meristele (fig. G, x) resembling that of _P. punctata_. Adventitious roots are seen in section at R (figs. G and H).
=Polypodiaceae.=
Sections of petrified sporangia from the English Coal-Measures (_Pteridotheca_ sp.) occasionally exhibit a striking resemblance to those of recent Polypodiaceae[927], but in the absence of material in which it is possible to recognise the true orientation of the sporangia, the exact position of the annulus is almost impossible to determine. We have as yet no satisfactory evidence of the existence of true Polypodiaceae in the Palaeozoic era. It is noteworthy that apart from the absence of ferns which can reasonably be included in this family, the anatomical features of the Botryopterideae (Coenopterideae) and of the Cycadofilices or Pteridosperms do not foreshadow those of Polypodiaceous ferns. On the other hand, as we have already noticed, anatomical characters of such families as the Gleicheniaceae, Hymenophyllaceae, and Schizaeaceae are met with in certain generalised Palaeozoic types. These facts are perhaps of some importance as supplying collateral evidence in favour of the relatively more recent origin of the dominant family of ferns in modern floras.
[Illustration: Fig. 279. A. _Adiantides antiques_ (Ett.). (½ nat. size.) B. _A. Lindsayoides_ (Sew.). (B′ nat. size.)
(A, after Kidston.)]
The use of the generic name _Adiantites_ for fern-like fronds of Lower Carboniferous age characterised by cuneate pinnules like those of species of _Adiantum_, suggests an affinity which is in all probability non-existent. It has been pointed out that this generic name was applied in the first instance to the leaves of the Jurassic plant _Ginkgo digitata_[928] and should, therefore, be discarded. Schimper[929] used the designation _Adiantides_, and Ettingshausen[930], more rashly than wisely, preferred _Adiantum_. The specimens described by Kidston[931] as _Adiantides antiquus_ (Ett.) (fig. 279, A) from the Carboniferous limestone of Flintshire are portions of tripinnate fronds bearing cuneate segments with numerous forked veins radiating from the contracted base of the lamina. It is not improbable, in view of Dr White’s[932] discovery of seeds on a very similar plant from the Pottsville beds of North America, that this characteristic Lower Carboniferous genus is a Pteridosperm.
From Jurassic rocks in various parts of the world numerous fossils have been described under the generic names _Aspidium_, _Asplenium_, _Davallia_, _Polypodium_, and _Pteris_. In the great majority of cases such records leave much to be desired from the point of view of students who appreciate the dangers of relying on external similarity between vegetative organs, and on resemblances founded on obscure impressions of sori. The generic term _Woodwardites_[933], which suggests affinity with the recent genus _Woodwardia_, has been used for Rhaetic plants belonging to the Dipteridinae.
A plant described as _Adiantides Lindsayoides_ from Jurassic rocks of Victoria[934], characterised by marginal sori which appear to be protected by the folded-over edge of the leaflets, and by the resemblance of the pinnules to those of recent species of _Lindsaya_, may be a true Polypodiaceous fern; but in this case, as in many similar instances, nothing is known of the structure of the sporangia. Some sterile pinnae described by Yabe from Jurassic rocks of Korea as _Adiantites Sewardi_[935] may perhaps be identical with the Australian species.
In such a species as _Polypodium oregonense_ Font., from Jurassic rocks of Oregon, the generic name is chosen because the “fructification seems near enough to that of _Polypodium_ to justify the placing of the plant in that genus[936].” But the fact that no sporangia have been found is a fatal objection to this identification.
_Onychiopsis._
This generic name was instituted by Yokoyama[937] for a Japanese Wealden species, previously described by Geyler[938] as _Thyrsopteris elongata_, on the ground that, in addition to a similarity in habit of the sterile fronds, the fertile pinnae present a close agreement to those of the recent genus _Onychium_.
_Onychiopsis Mantelli_[939] (Brongn.). Figs. 278, D; 280, A and B.
The Japanese species _Onychiopsis elongata_ may perhaps be identical with this common Wealden fern which, as Fontaine points out, should be called _O. psilotoides_ if the rule of priority is to be observed irrespective of long usage.
1824. _Hymenopteris psilotoides_, Stokes and Webb, Trans. Geol. Soc. [ii.], Vol. I. p. 423, Pl. XLVI. fig. 7.
1828. _Sphenopteris Mantelli_, Brongniart, Hist. vég. foss. p. 170, Pl. XLV. figs. 3–7.
1890. _Onychiopsis Mantelli_, Nathorst, Denksch. Wien Akad. Vol. LVII. p. 5.
_Onychiopsis Mantelli_ may be defined as follows:—
Frond bipinnate, ovate lanceolate, rachis winged; pinnae approximate, given off at an acute angle; pinnules narrow, acuminate, with a single vein; the larger segments serrate and gradually passing into pinnae with narrow ultimate segments. Fertile segments sessile or shortly stalked, linear ovate, sometimes terminating in a short awn-like prolongation.
The fertile segments (fig. 278, D) bear so close a resemblance to those of species of _Onychium_ that it would seem justifiable to regard the plant as a member of the Polypodiaceae. This fern is one of the most characteristic members of the Wealden floras; it occurs in abundance in the English Wealden, in Portugal, Germany, Belgium, Japan, Bohemia, South Africa, and elsewhere. A piece of rhizome figured from the English Wealden[940] is very similar to the creeping rhizomes of recent species of Polypodiaceae. The English Wealden specimens shown in fig. 280, A and B, illustrate the difference in form presented by leaves of this species; the smaller pinnae reproduced in fig. A are more characteristic of the species than are those of the slightly enlarged example represented in fig. 280, B.
[Illustration: FIG. 280. _Onychiopsis Mantelli._ (From Wealden specimens in the British Museum; No. 13495 and No. V. 2615. A, natural size; B, very slightly enlarged.)]
Among British Tertiary species referred to Polypodiaceae, it is interesting to find what may well be an authentic record of a fern closely allied to the recent tropical species _Acrostichum_ (_Chrysodium_) _aureum_. This Eocene species from Bournemouth is described as _Chrysodium lanzaeanum_[941]. The frond is simply pinnate and apparently coriaceous in texture, with lanceolate or oblong lanceolate pinnules (fig. 261, A, A′, p. 350), differing from those of _Acrostichum aureum_ in being sessile. A prominent midrib gives off numerous anastomosing veins. No fertile pinnules have been found.
Specimens described by Forbes from the Eocene beds of the Island of Mull as _Onoclea hebraidica_[942] bear a strong likeness to the North American and Japanese recent species _Onoclea sensibilis_. Fertile specimens referred to the latter species are recorded by Knowlton[943] from Tertiary beds of Montana.
A species described by Saporta[944] from the Eocene of Sézanne as _Adiantum apalophyllum_ is recorded by Gardner and Ettingshausen from Bournemouth; an identification which is based on somewhat meagre evidence.
The following remarks by Gardner and Ettingshausen are worthy of repetition as calling attention to circumstances often overlooked in analyses of fossil floras. They speak of ferns as relatively rare in British Eocene rocks and add,—“the floras consist principally of deciduous dicotyledonous leaves, which ... fell into the water and were tranquilly silted over. Ferns, on the other hand, would require some violence to remove them from the place of their growth, and their preservation would consequently be exceptional, and they would be mutilated and fragmentary. This may account for their rarity. Few as the British ferns are in the number of species, they nevertheless form the largest and most important series of Eocene ferns, even of Tertiary ferns, yet described from one group of beds[945].”
=Dipteridinae.=
_Dictyophyllum._
This genus was founded by Lindley and Hutton for a pinnatifid leaf from the Jurassic rocks of Yorkshire which they regarded as probably dicotyledonous and named _D. rugosum_[946]. Several ferns of this genus have since been found with well-preserved sori which demonstrate a close similarity to the recent fern _Dipteris._ _Dictyophyllum_ may be defined as follows:—
Fronds large and palmate, characterised by the equal dichotomy of the main rachis into two arms which curve outwards and then bend inwards (fig. 281); from the surface of each arm are given off numerous spreading pinnae with a lamina more or less deeply dissected into lobes varying in breadth and in the form of the apex. Each lobe has a median vein, from which branches are given off approximately at right angles and then subdivide into a reticulum, in the meshes of which the veinlets end blindly (fig. 282, A and E). Sori composed of annulate sporangia are crowded on the lower surface of the lamina. In habit and in sporangial characters the genus closely resembles _Dipteris_, and in the branching of the frond suggests comparison with _Matonia_. The rhizome (_Rhizomopteris_) is creeping and dichotomously branched, bearing leaf-scars with a horse-shoe form of vascular strand.
[Illustration: FIG. 281. _Dictyophyllum exile._ (After Nathorst; much reduced.)]
_Dictyophyllum_ is represented by several types to which various specific names have been assigned, the distinguishing features being the form of the pinna lobes, the degree of concrescence between the basal portions of the pinnae, and similar features which in some cases can only be safely used as criteria when large specimens are available for comparison.
_Dictyophyllum exile_ (Brauns). Figs. 281, 282, D, E.
1862. _Camptopteris exilis_, Brauns, Palaeontograph. IX. p. 54.
1867. _Dictyophyllum acutilobum_, Schenk, Foss. Flor. Grenz. p. 77, Pls. XIX. XX.
1878. _D. exile_, Nathorst, Flora vid Bjuf, I. p. 39, Pl. V. fig. 7.
—— _D. acutilobum_, _ibid._ Pl. XI. fig. 1.
The restoration, after Nathorst[947], shown in fig. 281 illustrates the habit of this striking fern, examples of which or of closely allied species are recorded from Rhaetic rocks of Germany, Scania, Persia, Bornholm, Tonkin, China, and elsewhere[948]. The petiole, reaching a length of 60 cm., forks at the apex into two equal arms leaving between them an oval space and occasionally crossing one another. The axes of these branches are twisted so that the pinnae, which may be as many as 24 on each arm, and arise from the inner side, by torsion of the axes assume an external position. An interesting analogy as regards the twisted rachis of _Dictyophyllum exile_ and _Camptopteris_ is afforded by the leaves of the Cycads, _Macrozamia Fawcettiae_ and _M. corallipes_, which are also characterised by the torsion of the rachis. The habit, justly compared by Nathorst with that of _Matonia pectinata_, affords another illustration of the common occurrence in older ferns of a dichotomous system of branching. The pinnae, characterised by circinate vernation, reach a length of 60 cm. and are divided into linear lobes inclined obliquely or at right angles to the pinna axis. The whole of the under surface of the lamina may be covered with sporangia, 4–7 sporangia in each sorus; the annulus is incomplete and approximately vertical (fig. 282, D). The rhizome is probably represented by the dichotomously branched axis described by Nathorst from Scania as _Rhizomopteris major_; the leaf-scars show a horse-shoe leaf-trace.
[Illustration: FIG. 282.
A. _Dictyophyllum Nilssoni._ B. _Rhizomopteris Schenki._ C. _Camptopteris spiralis._ D, E. _Dictyophyllum exile._
(After Nathorst; A, B, C, E, ⅔ nat. size.)]
_Dictyophyllum Nathorsti_ Zeiller[949].
This type, represented by a splendid series of specimens from the Rhaetic beds of Tonkin, agrees very closely with _D. exile_. It differs, however, in the basal parts of the pinnae which are concrescent for a length of 5 to 8 cm. instead of free as in _D. exile_; and, to a slight degree, in the form of the ultimate segments. In habit and in soral characters the two species are practically identical. Each sorus contains 5 to 8 sporangia, which are rather larger than those of _Dipteris_.
_Dictyophyllum rugosum_, Lind. and Hutt. Fig. 283.
1828. _Phlebopteris Phillipsii_, Brongniart, Hist. vég. foss. p. 377, Pl. CXXXII. fig. 3; Pl. CXXXIII. fig. 1.
1829. _Phyllites nervulosis_, Phillips, Geol. Yorks. p. 148, Pl. VIII. fig. 9.
1834. _Dictyophyllum rugosum_, Lindley and Hutton, Foss. Flor. II. Pl. CIV.
1836. _Polypodites heracleifolius_, Goeppert, Foss. Farn. p. 344.
1849. _Camptopteris Phillipsii_, Brongniart, Tableau, p. 105.
1880. _Clathropteris whitbyensis_, Nathorst, Berättelse, p. 83.
This species, which is characteristic of Jurassic rocks, is less completely known than the two types described above, but in the form and venation of the pinnae there is little difference between the Rhaetic and Jurassic plants. The leaves of the Jurassic species appear to have been smaller and more like those of _Dipteris conjugata_ (fig. 231); there are no indications of the existence of the two curved arms at the summit of the petiole which form so striking a feature in _D. exile_ and _D. Nathorsti_. No sporangia have been found on English specimens, but it is safe to assume their agreement with those of other species. A more complete list of records of _D. rugosum_ is given in the first volume of the British Museum Catalogue of Jurassic plants[950].
[Illustration: FIG. 283. _Dictyophyllum rugosum_ (Lind. and Hutt.). (Brit. Mus. Nat. size.)]
Nathorst[951] has recently drawn attention to certain differences between _Dictyophyllum_ and _Dipteris_. The pinnate division of the pinnae is not represented in the fronds of the recent species, but this method of lobing, which is a marked characteristic of _Dictyophyllum_, is less prominent in _Clathropteris_; and in _Camptopteris lunzensis_ Stur[952], an Austrian Upper Triassic species, the pinnae are entire. In _Dictyophyllum_ the sori cover the whole lower surface of the leaf; in _Dipteris_ they are more widely separated and the sporangia have a diameter of 0·02 mm., but in _Dictyophyllum_ the diameter is 0·4–0·6 mm. Moreover in _Dictyophyllum_ the sori contain 5 to 8 sporangia, whereas in _Dipteris_ they are much more numerous. Despite these differences it is clear, as Nathorst says, that _Dictyophyllum_, _Clathropteris_, and _Camptopteris_ are existing types very closely allied to _Dipteris_. It is a matter of secondary importance whether we include all in the Dipteridinae or follow Nathorst’s suggestion and refer the fossil genera to the separate family Camptopteridinae.
_Thaumatopteris._
This genus, founded by Goeppert[953] for a Rhaetic plant from Bayreuth, is by some authors[954] regarded as identical with _Dictyophyllum_, but it has recently been resuscitated by Nathorst[955] for specimens which he names _T. Schenki_, formerly included by Schenk in his species _T. Brauniana_[956]. It bears a close resemblance, in the long linear pinnules with an entire or crenulate margin, to _Dictyophyllum Fuchsi_ described by Zeiller[957] from Tonkin, and it would seem hardly necessary to adopt a distinctive generic designation. The sporangia have a vertical or slightly oblique annulus and the rhizome is similar to that of _Dictyophyllum exile_. The habit of the genus is shown in fig. 284, which represents one of the German Rhaetic species.
[Illustration: FIG. 284. _Thaumatopteris Münsteri._ (From a specimen in the Bergakademie, Berlin; ⅓ nat. size.)]
_Clathropteris._
_Clathropteris meniscoides_, Brongn. Fig. 285.
_Clathropteris_, founded by Brongniart[958] for Rhaetic specimens from Scania, agrees very closely with some species of _Dictyophyllum_, but in view of the more rectangular form of the venation-meshes it is convenient to retain both names. The type-species was originally named _Filicites meniscoides_[959] and afterwards transferred to _Clathropteris_. An examination of Brongniart’s specimens has convinced Nathorst of the specific identity of _C. meniscoides_ and _C. platyphylla_. The Tonkin leaves described by Zeiller[960] under the latter name should, therefore, be included in _C. meniscoides_, which may be thus defined:
The petiolate frond is characterised by an equal dichotomy of the rachis, as in _Dictyophyllum_; each branch bore 5–15 pinnae, disposed _en éventail_, reaching a length of 20–30 cm. and fused basally as in _D. Nathorsti_ Zeill. Pinnae linear lanceolate, slightly contracted at the lower end and gradually tapered distally. The lamina, 3–14 cm. broad, is characterised by obtusely pointed marginal lobes. From the midrib of each pinna lateral veins are given off at a wide angle, and adjacent veins are connected by a series of branches which divide the lamina into a regular reticulum of rectangular and polygonal meshes (fig. 285). The sori are abundant and contain 5–12 sporangia like those of _Dictyophyllum_.
[Illustration: FIG. 285. _Clathropteris meniscoides._ From Rhaetic rocks near Erlangen. [M.S.]]
[Illustration: FIG. 286. _Clathropteris egyptiaca._ (Nat. size.) _a_, _b_, pieces of main ribs in grooves.]
What is probably the rhizome of this species has been described by Nathorst (_Rhizomopteris cruciata_); it is similar to that of _Dictyophyllum_, but the leaf-scars are more widely separated. This species occurs in Upper Triassic, Rhaetic or Lower Jurassic rocks of Scania, France, Germany, Switzerland, Bornholm, North America, China, Tonkin, and Persia and is represented by fragments in the Rhaetic beds of Bristol[961].
_Clathropteris egyptiaca_ Sew.[962] Fig. 286.
The specimen on which this species was founded was discovered in the Nubian Sandstone east of Edfu; the age of the beds is uncertain, but the presence of _Clathropteris_ suggests a Lower Jurassic or Rhaetic horizon[963]. Seven strong ribs radiate through the lamina from the summit of the petiole; at _a_ and _b_ small pieces of the projecting ribs are shown in the grooves. From the main veins slender branches are given off at right angles and, as seen in the enlarged drawing, these again subdivide into a delicate reticulum with free-ending veinlets.
[Illustration: FIG. 287. _Camptopteris spiralis._ (After Nathorst. Much reduced.)]
_Camptopteris._
_Camptopteris spiralis_, Nath. Figs. 282, C; 287.
Nathorst proposed this generic name for Rhaetic fronds[964] resembling those of _Clathropteris_ and _Dictyophyllum_, but differing in the form of the pinnae and in habit. The habit of the type-species, _C. spiralis_, is shown in fig. 287. An examination of the specimens in the Stockholm Museum convinced me of the correctness of Nathorst’s restoration[965]. Each of the forked arms of the rachis bore as many as 150–160 long and narrow pinnae characterised by an anastomosing venation (fig. 282, C) and by a spiral disposition due to the torsion of the axes. The sporangia agree in essentials with those of _Dictyophyllum_.
_Hausmannia._
A critical and exhaustive account of this genus has been given by Prof. Von Richter[966] based on an examination of specimens found in the Lower Cretaceous rocks of Quedlinburg in Germany. The name was proposed by Dunker[967] for leaves from the Wealden of Germany characterised by a deeply dissected dichotomously branched lamina. Andrae subsequently instituted the genus _Protorhipis_[968] for suborbicular leaves with dichotomously branched ribs from the Lias of Steierdorf. A similar but smaller type of leaf was afterwards described by Zigno[969] from Jurassic beds of Italy as _P. asarifolius_, and Nathorst[970] figured a closely allied form from Rhaetic rocks of Sweden. While some authors regarded _Hausmannia_ and _Protorhipis_ as ferns, others compared them with the leaves of _Baiera_ (Ginkgoales); Saporta suggested a dicotyledonous affinity for leaves of the _Protorhipis_ type. The true nature of the fossils was recognised by Zeiller[971], who called attention to the very close resemblance in habit and in soral characters to the recent genus _Dipteris_. A comparison of the different species of _Dipteris_, including young leaves (fig. 231, p. 297), with those of the fossil species reveals a very striking agreement[972]. There can be no doubt, as Richter points out, that the names _Hausmannia_ and _Protorhipis_ stand for one generic type.
_Hausmannia_ may be defined as follows:
Rhizome creeping, slender, dichotomously branched; leaf-stalks slender (2–25 cm. long), bearing a leathery lamina (1–12 cm. long and broad), wedge-shaped below, occasionally cordate or reniform, entire or more or less deeply lobed into broad linear segments. The leaf is characterised by dichotomously branched main ribs which arise from the summit of the rachis as two divergent arms and radiate in a palmate manner, with repeated forking, through the lamina. Lateral veins are given off at a wide angle, and, by subdivision, form a fairly regular network similar to that in _Dictyophyllum_, _Clathropteris_, and _Dipteris_.
[Illustration: FIG. 288. _Hausmannia dichotoma._ (Specimens from the late Dr Marcus Gunn’s Collection of Upper Jurassic plants, Sutherlandshire; very slightly reduced.)]
_Hausmannia dichotoma_, Dunker[973]. Fig. 288, A, B.
This Wealden species, represented in the North German flora and in beds of approximately the same age at Quedlinburg, has been discovered by Dr Marcus Gunn in Upper Jurassic rocks on the north-east coast of Scotland. The lamina (12 cm. or more in length) is divided into five to seven linear segments and bears a close superficial resemblance to leaves of _Baiera_ and to recent species of _Schizaea_ (fig. 222, p. 287). Each segment contains one or two main ribs (fig. 288, A). A similar form is described by Bartholin[974] and by Moeller[975] as _H. Forchammeri_ from Jurassic rocks of Bornholm.
_Hausmannia Kohlmanni_, Richt. Fig. 278, F.
In this species, instituted by Richter from material obtained from the Lower Cretaceous beds of Strohberg[976], the comparatively slender rhizome bears fronds with petioles reaching a length in extreme cases of 25 cm. but usually of about 10 cm. The lamina (1–7 cm. long and 1–10 cm. broad) is described as leathery, obcordate, and divided into two symmetrical halves by a median sinus which, though occasionally extending more than half-way through the lamina, is usually shallow. The venation consists of two main branches which diverge from the summit of the petiole (fig. 278, F) and subdivide into dichotomously branched ribs; finer veins (not shown in the drawing) are given off from these at right angles and form more or less rectangular meshes as in other members of the Dipteridinae and in such recent ferns as _Polypodium quercifolium_ (fig. 231, D, p. 297).
The imperfect lamina represented in fig. 289 may belong to _Hausmannia Richteri_ or may be a distinct species; it shows some of the finer veins connecting the shorter forked ribs, which formed part of the reticulate ramifying system in the mesophyll. This specimen was obtained from the plant-beds of Culgower on the Sutherlandshire coast, which have been placed by some geologists in the Kimmeridgian series.
The smaller type represented in fig. 278, E, is referred by Richter to a distinct species, _Hausmannia Sewardi_[977], founded on a few specimens from the Lower Cretaceous strata of Strohberg. This species is characterised by a stouter rhizome bearing smaller leaves consisting of a short petiole (3–4 cm. long) and an obovate lamina (1–2 cm. long and broad). There are usually two opposite leaflets on each leaf-stalk, and these may be equivalent to the two halves of a single deeply dissected lamina.
[Illustration: FIG. 289. _Hausmannia sp._ Upper Jurassic, near Helmsdale, Scotland. From a specimen in the British Museum. (Nat. size.)]
It is interesting to compare these different forms of _Hausmannia_ with the fronds of recent species of _Dipteris_ represented in fig. 231. The more deeply dissected type, such as _H. dichotoma_, closely resembles _D. Lobbiana_ or _D. quinquefurcata_, while the more or less entire fossil leaves (fig. 278, E, F and fig. 289) are very like the somewhat unusual form of _Dipteris conjugata_ shown in fig. 231, B, p. 297.
Other species of the genus are recorded from Liassic rocks of Steierdorf[978] (Hungary) and of Bornholm[979]. Nathorst[980] has described a small Rhaetic species from Scania: a French Permian plant described by Zeiller[981] and compared by him with _H. dichotoma_, may be a Palaeozoic example of this Dipteris-like genus.
Some segments of leaves from the Eocene beds (Middle Bagshot) of Bournemouth, and now in the British Museum, described by Gardner and Ettingshausen[982] as _Podoloma polypodioides_, bear a close resemblance in the venation to the lamina of _Dipteris conjugata_.
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