chapter XXII

., the species _Pecopteris arborescens_ may be briefly described.

_Pecopteris arborescens_ (Schlotheim)[1584]. Figs. 352, D: 376.

The species named by Schlotheim _Filicites arborescens_ in 1804 is characteristic of the Upper Coal-Measures and is recorded also from Permian strata[1585].

Fronds large; the rachis, which may reach a breadth of 3 cm.[1586], gives off long ovoid-lanceolate pinnae in two alternate rows (fig. 376); pinnules small, 1·5–4mm. long and 1–2mm. broad, contiguous, with rounded apex, attached approximately at right angles; the upper surface of the lamina is slightly convex and may be hairy[1587]. The fertile pinnules, identical in shape with the sterile, bear groups of ovoid exannulate sporangia (synangia). The midrib extends to the apex of the pinnule and gives off simple veins at a wide angle (fig. 352, D).

Our knowledge of the reproductive organs is very meagre. Grand’Eury described the synangia as consisting of 3–5 sporangia borne on a central receptacle; sporangia have been described also by Stur[1588], Renault and Zeiller[1589], and Potonié[1590], but no fertile British specimens are recorded. Stur places this species in the genus _Scolecopteris_, and Potonié regards the sporangia found by him on Permian fronds, which may be identical with _Pecopteris arborescens_, as conforming to those of the _Asterotheca_ type. It is impossible to decide on the evidence available whether this species is a Pteridosperm or a fern, but there is a natural inclination in doubtful cases to give preference to the first of these two choices.

[Illustration: FIG. 376. _Pecopteris arborescens_ (Schloth.). From the Upper Coal-Measures of Radstock. From a photograph by Dr Kidston. Reduced.]

The numerous fronds from Carboniferous and Permian rocks described as species of _Pecopteris_ exhibit a considerable range of variation in the form of the pinnules. In many species the pinnules are of the type represented in fig. 352, D; in others the lamina of the ultimate segments is slightly contracted at the base and the secondary veins are given off at a more acute angle, as in _Pecopteris polymorpha_, Brongn.[1591] In _Pecopteris unita_, Brongn., already described as _Ptychocarpus unita_[1592], the pinnules are joined together except in the apical region. Some fronds included in _Pecopteris_ possess pinnules in which Pecopteroid and Sphenopteroid features are combined; _P. Sterzeli_, Zeill.[1593] and _P. Pluckeneti_, Schlot. are examples of fronds in which the pinnules are lobed as in _Sphenopteris_, but the base of the lamina is only slightly contracted and the venation is not that of typical Sphenopteris species.

The species to which Potonié has applied the generic name _Alloiopteris_[1594] also illustrates the impossibility of drawing a sharp line between _Pecopteris_ and _Sphenopteris_. The fronds already described in