CHAPTER XVI

CLASSES SCAPHOPODA AND PELECYPODA

CLASS SCAPHOPODA

Head rudimentary, mantle edges ventrally concrescent, forming a tube opening before and behind, and covered with a shell of the same shape; sexes separate.

[Illustration: FIG. 299.--Anatomy of _Dentalium_: _a_, anterior aperture of mantle; _f_, foot; _g_, genital gland; _k_, kidney; _l_, liver. (After Lacaze-Duthiers.)]

The Scaphopoda form a small but very distinct class, whose organisation is decidedly of a low type. The body is usually slightly curved, the concave side being the dorsal; muscles near the posterior end attach the body to the shell. The foot, which can be protruded from the anterior or wider aperture, is rather long, pointed, and has sometimes two lateral lobes (_Dentalium_), sometimes a terminal retractile disc (_Siphonodentalium_), sometimes a retractile disc with a central tentacle (_Pulsellum_). The cephalic region, as in Pelecypoda, is covered by the mantle. The mouth is situated on a kind of projection of the pharynx; the buccal mass, containing the radula (p. 236), is at the base of the foot, and the intestine branches forward from the front part of the stomach. The liver (Fig. 299) is paired, and consists of a number of symmetrical, radiating coeca. There are no eyes, but on each side of the mouth are small bunches of exsertile filaments (_captacula_), which appear to act as tactile organs for the seizing of food. There is no special respiring apparatus, heart or arterial system, breathing being conducted by the walls of the mantle. The nervous system has already been described (p. 205).

Two kidneys open on either side of the anus. The genital gland is large, occupying nearly all the posterior part of the body, the sexual products being emitted through the right kidney. The veliger has already been figured (p. 131, Fig. 44). The embryonic shell is formed of two calcareous laminae, which subsequently unite to form the tube.

With regard to their general relationships, the Scaphopoda resemble the Gasteropoda in their univalve shell, and in the possession of a radula; while the pointed foot, the non-lobed velum in the veliger, the generative system, the bilateral symmetry of the organs generally, and the absence of any definite head, eyes, or tentacles, are points which approximate them to the Pelecypoda.

The Scaphopoda are known from Devonian strata to the present time. They are found at a depth of a few fathoms to very deep water. The only three genera are _Dentalium_, _Siphonodentalium_ (subg. _Cadulus_), and _Pulsellum_, which differ in the structure of the foot, as described above.

CLASS PELECYPODA

Cephalic region rudimentary, mantle consisting of two symmetrical right and left lobes, covering the body and secreting a bivalve shell hinged at the dorsal margin; no radula, sexes usually separate. Reference has already been made to the reproductive system (p. 145), breathing organs (p. 164 f.), mantle (p. 172), nervous system (p. 205), digestive system (p. 237 f.), and nomenclature of the various parts of the shell (p. 269 f.).

The shape of the shell, in many Pelecypoda, involving as it does the position, size, and number of the adductor muscles, is probably due to mechanical causes, depending on the habits and manner of life of the individual genus. Thus in a typical dimyarian or two-muscled bivalve, _e.g._ _Mya_ (Fig. 300, A), the adductor muscles lie well towards each end of the long axis of the shell, with the hinge about midway between them. In this position they are best placed for effectually closing the valves, and since they are nearly equidistant from the axis of motion, _i.e._ from the hinge, they do an equal amount of work, and are about equal in size. But in a form like _Modiola_, where the growth of the shell is irregular in relation to the hinge-line, the anterior muscle is brought nearer and nearer to the umbones, where its power to do work, and therefore its size, becomes less and less. But the work to be done remains the same, and the posterior muscle has to do it nearly all; hence it moves farther and farther away from the hinge-line, and at the same time gains in size. In shells like _Ostrea_, _Pecten_, and _Vulsella_, the anterior muscle, having drawn into line with the hinge and the posterior muscle, becomes atrophied, while the posterior muscle, having double work to do, has doubled its size.[409]

[Illustration: FIG. 300.--Illustrating changes in the position and size of the adductor muscles according to the shape of the shell: =A=, _Mya_; =B=, _Modiola_; =C=, _Vulsella_. The upper dotted line shows the hinge-line, the lower connects the two muscles.]

The development of the foot, again, largely depends upon habits of life. It is well developed in burrowing forms, while in sessile genera (_Ostrea_, _Chama_, _Spondylus_) it becomes unnecessary and aborts. Even in _Pecten_, which does not become sessile, but has ceased to use the foot as an organ of progression, a similar result follows. Forms which burrow deeply often “gape” widely, sometimes at one end only, sometimes at both. _Venus_, _Donax_, _Tellina_, _Mactra_, which are shallow burrowers, do not gape; _Solen_, _Lutraria_, and to a less degree _Mya_, burrow deeply and gape widely. In order to burrow deeply the foot must be highly developed, and the larger it becomes, the more will it tend to keep the valves apart at the place where it is habitually protruded. Burrowing species always remain in communication with the surface by means of their siphons, the constant extension of which tends to keep the valves apart at the end opposite to the foot. Burrowing species, again, tend to burrow in such a way as to descend most easily, and not be impeded by their own shells; in other words, they act as a wedge, and descend with their narrowest part foremost. But the burrowing organ, the foot, has to follow suit, and gradually draws round to the narrowest part of the shell, so that the habitual deep burrower, such as _Lutraria_, lies with its long axis exactly at right angles to the surface, its siphons protruding from, and keeping open, the uppermost or posterior margin of the shell, and the foot producing the same effect upon the lower or anterior margin. The deeper the burrower, the more elongated does the shell become, until, through forms like _Pholas_ and _Saxicava_, we arrive at _Solen_, the most highly specialised burrower of all, in which the breadth of the shell is equal throughout, and no obstructive curve exists to impede its rapid ascent or descent.

The Pelecypoda have been classified in various ways; by the completeness or sinuation of the pallial line, depending on the absence or presence of siphons, by the number of adductor muscles, by the character of the hinge-teeth, and by the number of the branchiae. For various reasons, none of these methods have proved entirely satisfactory. That adopted here was suggested by Pelseneer, and depends upon the character of the branchiae themselves, as suggesting successive stages of development (p. 166 f.).

=Order I. Protobranchiata=

Branchial filaments not reflected, the two rows inclined at a right angle (more or less), ventral surface of foot more or less flattened, byssogenous apparatus little developed, a single anterior aorta, kidneys distinct, sexes separate, each genital gland opening into the corresponding kidney.

FAM. 1. _Nuculidae._--Labial palps very large, rows of branchial filaments at right angles to one another, mantle edges open, siphons contracted, foot disc-shaped, elongated; shell equivalve, oval, or produced, interior generally nacreous, hinge with numerous saw-like teeth. Silurian ----. Principal genera: _Nucula_ (heart dorsal to the rectum); _Palaeoneilo_ (Devonian), (?) _Sarepta_, _Leda_, _Yoldia_, _Malletia_; _Tyndaria_ (Upper Tertiary), _Lyrodesma_ (Silurian), _Actinodonta_ (Silurian), _Babinka_ (Silurian).

FAM. 2. _Solenomyidae._--Labial palps united, one row of branchial filaments pointing dorsally, the other ventrally; mantle edges in great part united postero-ventrally, a single siphonal orifice with two very long tentacles, foot proboscidiform, with a round denticulate disc at the end; shell equivalve, resembling a Solen, with a strong corneous periostracum; no hinge-teeth, ligament internal. Single genus, _Solenomya_. (?) Cretaceous ----].

=Order II. Filibranchiata=

Rows of branchial filaments parallel, pointing ventrally, reflected, and provided with interfilamentary ciliated junctions, foot usually with a well-developed byssogenous apparatus.

=Sub-order I.= =Anomiacea.=--Heart dorsal to the rectum, a single aorta, foot small, anterior adductor very small; shell ostreiform, no hinge-teeth, fixed by a calcified byssus traversing the right valve (Fig. 173, p. 262).

FAM. _Anomiidae._--Jurassic ----. Genera: _Anomia_, _Placunanomia_; _Carolia_ (Eocene), _Placuna_; _Hypotrema_ (Jurassic), _Placunopsis_ (Oolite).

=Sub-order II.= =Arcacea.=--Mantle edge open, both adductors well developed, heart with two aortae, branchiae free, without interlamellar junctions, no siphons; renal and generative apertures distinct.

FAM. 1. _Arcadae._--Mantle edge with composite eyes; shell round or trapezoidal, solid, often with stout bushy periostracum; ligament often external, on a special area; hinge with numerous lamelliform teeth. Ordovician ----. Principal genera: _Arca_ (incl. _Barbatia_, _Scaphula_, and _Cucullaea_), heart dorsal to rectum; _Pectunculus_, _Glomus_, _Limopsis_; _Trinacria_ and _Nuculina_ (Tertiary).

FAM. 2. _Trigoniidae._--Foot large, hatchet-shaped, with ventral disc; no byssus, mantle edge with ocelli; shell sub-trigonal, hinge-teeth few, strong; interior violet-nacreous. Devonian ----. Genera: _Trigonia_; _Myophoria_ and _Schizodus_ (Trias), _Cyrtonotus_ (Devonian).

[Illustration: FIG. 301.--_Trigonia pectinata_ Lam., Sydney, N.S.W.]

=Sub-order III.= =Mytilacea.=--Mantle edges fused at one point, anal orifice distinct, anterior terminal adductor small, one aorta, branchiae with interfoliary junctions, genital glands penetrating the side of the mantle and opening by the side of the kidneys.

FAM. _Mytilidae._--Byssus well developed, shell more or less equivalve, oval, broad; hinge-teeth evanescent. Devonian----. Principal genera: _Mytilus_?, _Myalina_, _Septifer_, _Modiola_, _Lithodomus_, _Crenella_, _Dacrydium_, _Myrina_, _Idas_, _Modiolaria_, _Modiolarca_.

=Order III. Pseudolamellibranchiata=

Mantle edges entirely open, foot little developed, anterior adductor usually aborted, branchial filaments reflected, with interlamellar junctions, which are sometimes vascular; genital glands opening into the kidneys or close to the apertures of the kidneys.

FAM. 1. _Aviculidae._--Foot long, tongue-shaped, byssogenous apparatus well developed, branchiae concrescent with the mantle, adductor muscle sub-central, at times a small anterior adductor, siphons absent; shell usually inequivalve, dorsal margin straight, often very long, winged, lateral teeth much prolonged; structure of shell cellular, inside prismatic, outside nacreous. Palaeozoic ----. Principal genera: _Avicula_, including _Meleagrina_, _Malleus_; _Vulsella_ (no wings or hinge-teeth); _Perna_, including _Crenatula_, _Inoceramus_ (ligaments in a number of fossettes); _Aucella_ and _Monotis_ (Palaeozoic and Secondary); _Pterinaea_ and _Ambonychia_ (Palaeozoic); _Pinna_; _Aviculopinna_ (Carboniferous).

[Illustration: FIG. 302.--_Avicula heteroptera_ Lam., Australia, showing the inequivalve shell and byssal sinus (_bs_).]

FAM. 2. _Prasinidae._--Shell very small, umbones anterior, incurved, anterior side depressed, hinge-teeth replaced by dentiform projections of the lunule fitting into corresponding grooves. Recent. Single genus, _Prasina_.

FAM. 3. _Ostreidae._--Heart generally ventral to the rectum, branchiae concrescent with the mantle, no byssus; shell inequivalve, fixed by the left valve, form irregular. Jurassic ----. Genera: _Ostrea_; _Heligmus_ (Oolite), _Naiadina_ (Cretaceous), _Pernostrea_ (Jurassic).

FAM. 4. _Pectinidae._--Byssus usually absent, mantle edge open, duplicated, folded back, with pallial ocelli; branchiae not concrescent with the mantle; shell with unequal “ears” at the umbo, hinge-teeth lamelliform, often obscure. Silurian----. Principal genera: _Pedum_, _Chlamys_, _Hinnites_, _Hemipecten_, _Amussium_, _Pecten_; _Aviculopecten_ (Palaeozoic), _Crenipecten_.

FAM. 5. _Limidae._--Mantle edge as in _Pecten_, tentaculate; shell sub-equivalve, eared, fixed by a byssus or free. Carboniferous----. Genera: _Lima_ (Fig. 85, p. 179), _Limea_.

FAM. 6. _Spondylidae._--Foot with a peduncular appendage, no byssus, numerous pallial ocelli; shell fixed by right valve, surface often very spinose, two cardinal teeth in each valve. Jurassic----. Genera: _Plicatula_, _Spondylus_; _Terquemia_ (Lias).

[Illustration: FIG. 303.--_Pecten pallium_ L., East Indies.]

[Illustration: FIG. 304.--_Spondylus petroselinum_ Sowb., Mauritius; on a coral.]

FAM. 7. _Dimyidae._--Shell ostreiform, fixed, hinge with or without symmetrical teeth, two muscular impressions. Single genus, _Dimya_ (Tertiary).

=Order IV. Eulamellibranchiata=

Mantle edges united at one or more points, branchiae with interfilamentary junctions which are always vascular, genital glands not opening into the kidneys, usually two adductor muscles.

=Sub-order I. Submytilacea.=--Mantle edges more or less open, anal orifice distinct, usually no siphons, pallial line usually simple, cardinal and lateral teeth well marked.

FAM. 1. _Carditidae._--Foot with a byssus or groove, branchiae large, unequal; shell equivalve, solid, radiately grooved, one or two oblique cardinal teeth, one or two laterals. Silurian ----. Principal genera: _Venericardia_, _Cardita_, _Carditella_, _Carditopsis_, _Milneria_; _Pleurophorus_ (Palaeozoic), _Anodontopsis_ (Silurian).

FAM. 2. _Astartidae._--A short anal siphon, labial palps large; shell triangular, thick, ligament external, hinge with two or three cardinals in each valve, laterals obscure.? Devonian ----. Principal genera: _Astarte_; _Pachytypus_ (Jurassic), _Plesiastarte_ (Eocene), _Parastarte_, _Woodia_, _Opis_ (Secondary strata), _Prosocoelus_ (Devonian).

FAM. 3. _Crassatellidae._--Mantle with anal orifice or open; shell equivalve, thick, subtriangular, ligament in an internal fossette, hinge with two cardinals, laterals produced. Cretaceous----. Principal genus, _Crassatella_.

FAM. 4. _Cardiniidae._--Shell equivalve, oval or triangular, ligament external, cardinal teeth small, laterals fairly strong. Devonian----Oolite. Principal genera: _Cardinia_, _Anthracosia_, _Carbonicola_, _Anoplophora_.

FAM. 5. _Cyprinidae._--Anal and branchial orifices complete, papillose, foot thick; shell variable, equivalve, thick, umbones often spiral, hinge-teeth very variable, ligament external. Jurassic----. Principal genera: _Cyprina_; _Pygocardia_ (Crag), _Veniella_ (Cretaceous), _Venilicardia_ (Secondary strata), _Anisocardia_ (Jurassic), _Isocardia_, _Libitina_, _Coralliophaga_; _Basterotia_ (Eocene). The families _Pachydomidae_ (Palaeozoic) and _Megalodontidae_ (Palaeozoic--Secondary) are probably related to the Cyprinidae.

[Illustration: FIG. 305.--_Isocardia vulgaris_ Reeve, China.]

FAM. 6. _Aetheriidae._--Anal orifice complete, foot absent, labial palps large; shell irregular, free or fixed, no hinge-teeth. Fluviatile, recent only. Genera: _Aetheria_, _Mülleria_, _Bartlettia_.

FAM. 7. _Unionidae._--Foot large and thick, no byssus, anal siphon short, branchial orifice complete or not, siphon present or absent, embryo of certain groups passing through a _glochidium_ stage (p. 146); shell equivalve, sometimes very thick, nacreous within, hinge variable. Fluviatile. Jurassic----. Principal genera: _Unio_ (subg. _Arconaia_), _Monocondylaea_, _Pseudodon_, _Anodonta_, _Solenaia_, _Mycetopus_, _Mutela_, _Spatha_, _Hyria_, _Castalia_, _Leila_.

FAM. 8. _Dreissensiidae._--Both siphons prominent, foot tongue-shaped, byssiferous; shell mytiliform, with small internal septum. Genera: _Dreissensia_; _Dreissensiomya_ (Tertiary). The common _Dreissensia polymorpha_ Pall. was distributed over large parts of Europe in later Tertiary times. From unknown causes it died out, and has during the past two hundred years been regaining its position, migrating N. and W. from its original habitat, the Caspian, by the Volga and its Oka confluent.

FAM. 9. _Modiolopsidae_.--Shell mytiliform, ligament exterior, hinge-teeth small, rather numerous. Palaeozoic----. Principal genera: _Modiolopsis_, _Cyrtodonta_, _Mytilops_, _Ptychodesma_.

FAM. 10. _Lucinidae._--Anal orifice sometimes with a siphon, branchial orifice complete or not, sometimes a single branchia; foot very long, vermiform, no byssus, anterior adductor long; shell rounded, equivalve, blanched, hinge with two cardinals and two laterals in each valve, sometimes toothless, ligament more or less internal. Silurian----. Principal genera: _Lucina_, _Corbis_, _Axinus_, _Diplodonta_, _Montacuta_.

FAM. 11. _Ungulinidae._--Anal orifice complete, foot vermiform, no byssus, two branchiae; shell equivalve, subcircular, hinge-teeth variable, no laterals, adductor impressions long, continuing the pallial line. Tertiary----. Single genus, _Ungulina_.

FAM. 12. _Unicardiidae._--Shell equivalve, round or oval, cardinal shelf large, a single cardinal in each valve, ligament external. Carboniferous--Cretaceous. Genera: _Unicardium_, _Scaldia_, _Pseudedmondia_.

FAM. 13. _Kellyellidae._--Anal siphon prolonged, no marked branchial orifice; shell very small, oval or round, anterior lateral very strong, under the cardinal. Eocene----. Genera: _Kellyella_; _Allopagus_ and _Lutetia_ (Tertiary), _Turtonia_.

FAM. 14. _Erycinidae._--Mantle edges with three apertures, branchial orifice on the buccal margin, foot long, broadened, with a byssus, animal usually viviparous. Tertiary----. Genera: _Erycina_, _Kellia_, _Pythina_, _Lasaea_, _Lepton_.

FAM. 15. _Galeommidae._--Mantle edges more or less reflected over the shell, apertures and foot as in Erycinidae; shell thin, equilateral, hinge with few teeth or none. Tertiary----. Genera: Galeomma, _Scintilla_, _Sportella_, _Chlamydoconcha_, _Hindsiella_, _Ephippodonta_ (Fig. 32, p. 81).

FAM. 16. _Cyrenidae._--Siphons short, foot large, no byssus; shell equivalve, subtriangular, with periostracum, hinge with two or three cardinals, laterals present; animal hermaphrodite, viviparous. Fresh or brackish water. Jurassic----. Genera: _Cyrena_, _Corbicula_ (subg. _Batissa_, _Velorita_), _Sphaerium_ (= _Cyclas_), _Pisidium_, _Galatea_, _Fischeria_.

The families _Cyrenellidae_ (single genus, _Cyrenella_) and _Rangiidae_ (single genus, _Rangia_) are probably to be placed here.

=Sub-order II. Tellinacea.=--Siphons long, separate, foot and labial palps very large, pallial sinus deep, two adductor muscles.

FAM. 1. _Tellinidae._--External branchial fold directed dorsally, foot with byssogenous slit, but no byssus, branchiae small; shell compressed, equivalve, ligament external, at least two cardinals in each valve, laterals variable. Cretaceous----. Principal genera: _Tellina_ (with many sections), _Gastrana_.

FAM. 2. _Scrobiculariidae._--Animal as in _Tellina_; shell orbiculate or long oval, equivalve, hinge-teeth weak, ligament in an internal cavity. Tertiary----. Principal genera: _Scrobicularia_, _Syndosmya_, _Theora_, _Cumingia_, _Semele_.

FAM. 3. _Donacidae._--External branchial fold directed ventrally; shell equivalve, subtriangular, solid, smooth, two or three cardinals in each valve, laterals variable, ligament external. Jurassic----. Genera: _Donax_, _Iphigenia_, _Isodonta_.

[Illustration: FIG. 306.--_Tellina rastellum_ Hanl., East Indies.]

FAM. 4. _Tancrediidae._--Shell donaciform, ligament external, cardinals usually two in each valve, posterior laterals strong. Trias----. Genera: _Tancredia_ (Secondary strata), _Hemidonax_.

FAM. 5. _Cardiliidae._--Shell heart-shaped, hinge as in Mactridae, posterior adductor resting on a myophore or shelf. Single genus, _Cardilia_. Tertiary----.

FAM. 6. _Mesodesmatidae._--Mantle edges largely united, with three orifices, foot byssiferous or not; shell regular or irregular, usually one cardinal and strong lateral teeth. Tertiary----. Genera: _Mesodesma_, _Ervilia_.

FAM. 7. _Mactridae._--External branchial fold directed ventrally, siphons fused, foot tongue-shaped; shell equivalve, triangular-oval, hinge with ligament in an internal fossette, another portion external, a bifurcated cardinal tooth in the left valve, fitting into a branching tooth in the right valve, laterals present. Jurassic----. Genera: _Nactra_, _Harvella_, _Raëta_, _Eastonia_, _Heterocardia_, _Vanganella_.

=Sub-order III.= =Veneracea.=--Branchiae slightly folded, foot compressed, siphons generally short, pallial line variable, two adductor muscles.

FAM. 1. _Veneridae._--Siphons free or partly united, foot seldom byssiferous; shell solid, equivalve, hinge usually with three cardinal teeth, laterals variable. Jurassic----. Principal genera: _Cytherea_, _Circe_; _Grateloupia_ (Tertiary), _Meroe_, _Dosinia_ (= _Artemis_), _Cyprimeria_, _Cyclina_, _Venus_, _Clementia_, _Lucinopsis_; _Thetis_ (Cretaceous), _Tapes_, _Venerupis_.

FAM. 2. _Petricolidae._--Animal perforating rocks; shell oval, slightly gaping behind, two or three cardinals, no laterals, pallial sinus well marked. Recent----. Genera: _Petricola_, _Naranio_.

[Illustration: FIG. 307.--_Cytherea dione_ Lam., Peru.]

FAM. 3. _Glaucomyidae._--Siphons long, united, foot small; shell produced, thin, hinge with three cardinals, no laterals, pallial sinus well marked. Recent----. Genus, _Glaucomya_ (incl. _Tanysiphon_).

=Sub-order IV.= =Cardiacea.=--Branchiae much folded back, mantle edges with three apertures, foot cylindroidal, more or less produced, siphons present or absent, one or two adductor muscles, pallial line variable.

FAM. 1. _Cardiidae._--Siphons rather long, foot long, no byssus; shell equivalve, more or less radiately ribbed, hinge with one or two cardinals in each valve, laterals variable, ligament external, two adductors. Brackish water or marine. Devonian----. Genera: _Byssocardium_ and _Lithocardium_ (Tertiary), _Conocardium_ (Palaeozoic), _Cardium_ (with many sections, including _Hemicardium_), _Limnocardium_ (subg. _Didacna_, _Monodacna_, _Adacna_).

[Illustration: FIG. 308.--_Cardium_ (_Hemicardium_) _cardissa_ L., East Indies.]

FAM. 2. _Lunulicardiidae._--Shell equivalve, very inequilateral subtriangular, anterior margin short or truncated, with a deep lunule. Single genus, _Lunulicardium_ (Palaeozoic).

FAM. 3. _Tridacnidae._--Mantle orifices widely separated, foot short, byssiferous, no anterior adductor; shell equivalve, large, thick, usually gaping in front, one cardinal tooth and one or two posterior laterals in each valve, no pallial sinus. Miocene----. Genera: _Tridacna_, _Hippopus_. The muscular power of the great _Tridacna_ is immense. Once caught between their gaping valves, a man’s hand or foot can scarcely be withdrawn. Two valves of _T. gigas_ in the British Museum weigh respectively 154 and 156 lbs.

FAM. 4. _Chamidae._--Mantle orifices widely separated, foot short, no byssus, both adductors present, ovary invading the mantle lobes; shell fixed, irregularly inequivalve, umbones spiral, ligament external, cardinal teeth often a mere ridge, anterior lateral strong, nearly central, no pallial sinus. Jurassic----. Genera: _Chama_; _Diceras_ (Jurassic), attached by one umbo, umbones very prominent, teeth strong; _Heterodiceras_ (Jurassic), _Requienia_ (Cretaceous), left valve widely spiral, attached by the umbo, right valve small, fitting on the other as an operculum, teeth obsolete; _Toucasia_, _Apricardia_, _Matheronia_ (all Secondary strata).

[Illustration: FIG. 309.--=A=, _Requienia ammonea_ Goldf., Neocomian, × ½; =B=, _Hippurites cornu-vaccinum_ Goldf., Cretaceous, × ¼. _a_, right valve; _f_, point of fixture. (From Zittel.)]

The four succeeding families require special study in a work on Palaeontology.

FAM. 5. _Monopleuridae._--Shell very inequivalve, left valve operculiform, right conical or spiral, fixed at the apex, ligament prolonged in external grooves. Cretaceous----. Genera: _Monopleura_, _Valletia_.

FAM. 6. _Caprinidae._--Shell very inequivalve, thick, free or fixed by apex of right valve, which is spiral or conical, left valve spiral or not, often perforated by radial canals from the umbo to the free margin. Neocomian and Cretaceous----. Principal genera: _Plagioptychus_, _Caprina_, _Ichthyosarcolites_, _Caprotina_, _Polyconites_.

FAM. 7. _Hippuritidae_ (= _Rudistae_).--Shell very inequivalve, externally as in Caprinidae, umbo central in left valve, no ligament proper, left valve with strong hinge-teeth and grooves, two adductor impressions on prominent myophores, shell structure of the two valves differing. Cretaceous only. Single genus, _Hippurites_ (Fig. 309, B).

FAM. 8. _Radiolitidae._--Shell inversely conical, biconical, or cylindrical, general aspect of _Hippurites_, umbo of left valve central or lateral, right valve with a thick outer layer, often foliaceous, umbonal cavity partitioned off by laminae. Cretaceous only. Genera: _Radiolites_, _Biradiolites_.

=Sub-order V.= =Myacea.=--Branchiae much folded back, mantle edges usually with three openings, foot compressed, siphons large, united or not, two adductor muscles, pallial line variable.

FAM. 1. _Psammobiidae._--Siphons long, not united, foot large, not byssiferous; shell equivalve, long, oval, slightly gaping at the ends, ligament external, prominent, two cardinal teeth in each valve, no laterals, a deep pallial sinus. Jurassic----. Genera: _Psammobia_, _Solenotellina_, _Sanguinolaria_, _Asaphis_, _Elizia_, _Quenstedtia_ (Jurassic).

FAM. 2. _Myidae._--Pedal orifice small, siphons long, united in great part; shell inequivalve, gaping at one or both ends, periostracum more or less extensive, ligament internal, resting on a prominent shelf; hinge-teeth variable. Cretaceous----. Genera: _Mya_, _Tugonia_, _Sphenia_, _Corbula_, _Lutraria_ (for which latter some propose a separate family).

FAM. 3. _Solenidae._--Foot long, powerful, more or less cylindrical, no byssus, siphons usually short, united or not, branchiae narrow; shell equivalve, long and narrow, gaping at both ends, with periostracum, umbones flattened, ligament external, hinge-teeth variable.? Devonian----. Genera: _Solecurtus_, _Pharella_, _Pharus_, _Cultellus_, _Siliqua_, _Ensis_, _Solen_, _Orthonota_ (?), _Palaeosolen_ (?).

FAM. 4. _Glycimeridae._--Pedal orifice very narrow, siphons long, united in great part, often covered with periostracum; shell more or less equivalve, gaping at both ends, hinge toothless or with two weak cardinals, ligament external; animal free or perforating. Cretaceous----. Genera: _Glycimeris_, _Saxicava_, _Cyrtodaria_.

FAM. 5. _Gastrochaenidae._--Foot small, cylindrical, no byssus, branchiae narrow, siphons long; shell perforating or cemented to a shelly tube, gaping widely on the anterior and ventral sides, no hinge-teeth, a deep pallial sinus. Cretaceous----. Genera: _Gastrochaena_, _Fistulana_ (tube with a median diaphragm, perforated by the siphons).

=Sub-order VI.= =Pholadacea.=--Mantle edges largely closed, siphons long, united, foot short, truncated, disc-shaped, ligament absent, two adductor muscles; animal perforating.

FAM. 1. _Pholadidae._--Organs contained within the valves, ctenidia prolonged into the branchial siphon, shell more or less gaping, thin, dorsal margin in part reflected over the umbones, one or more dorsal accessory pieces, no hinge-teeth, an interior apophysis proceeding from the umbonal cavity. Jurassic----. Genera: _Pholas_, _Talona_, _Pholadidea_ (posterior extremity of the valves prolonged by a corneous appendage, a passage to the long tube of _Teredo_), _Jouannetia_, _Xylophaga_, _Martesia_; _Teredina_ (Eocene).

[Illustration: FIG. 310.--_Teredo navalis_ L.: =V=, valves of shell; =T=, tube; =P=, pallets; =SS=, siphons. (After Möbius.)]

FAM. 2. _Teredinidae._--Animal vermiform, ctenidia mainly within the branchial siphon, siphons very long, with two calcareous appendages (“pallets”) near the anterior end, shell very small, continued into a long calcareous tube, valves deeply notched, internal apophysis as in Pholadidae. Lias----. Single genus, _Teredo_ (Fig. 310).

=Sub-order VII.= =Anatinacea.=--External branchial fold directed dorsally, not reflected, sexes united, ovaries and testes with separate orifices, mantle edges largely united, byssus usually absent, two adductor muscles, pallial line variable, shell usually nacreous within.

FAM. 1. _Pandoridae._--Siphons short, largely united, foot tongue-shaped; shell free or fixed, inequivalve, semi-lunar, or subtriangular, ligament often with calcareous ossicle, pallial line complete or with slight sinus. Cretaceous----. Genera: _Pandora_, _Myodora_, _Myochama_.

FAM. 2. _Chamostreidae._--Shell fixed, _Chama_-like, thick, umbones spiral, ligament with ossicle. Single genus, _Chamostrea_.

FAM. 3. _Verticordiidae._--Siphons not prolonged; shell heart-shaped, umbones prominent, spiral, ligament with an ossicle, pallial line complete. Miocene----. Genera: _Verticordia_, _Mytilimeria_, _Lyonsiella_.

FAM. 4. _Lyonsiidae._--Foot short, byssiferous, siphons short, separate, shell inequivalve, hinge-teeth usually absent, ligament and ossicle in an internal groove. Eocene----. Single genus, _Lyonsia_.

[Illustration: FIG. 311.--_Myochama Stutchburyi_ A. Ad., attached to _Circe undatina_ Lam., Moreton Bay.]

FAM. 5. _Ceromyidae._--Shell inequivalve, large, heart- or wedge-shaped, hinge toothless, ligament internal in one valve, external in the other. Secondary strata----. Genera: _Ceromya_, _Gresslya_.

FAM. 6. _Arcomyidae._--Shell equivalve, thin, surface finely granulated, hinge toothless, cardinal edge dentiform, ligament external. Secondary and Tertiary strata----. Genera: _Arcomya_, _Goniomya_, _Pleuromya_, _Machomya_.

FAM. 7. _Anatinidae._--A fourth (?byssal) pallial orifice, siphons long, separate or fused; shell thin, sometimes inequivalve, exterior often granulose, ligament often with ossicle, hinge toothless or with lamellae. Jurassic----. Genera: _Anatina_, _Plectomya_ (Secondary), _Periploma_, _Cochlodesma_, _Thracia_, _Tyleria_, _Alicia_, _Asthenothaerus_.

FAM. 8. _Grammysiidae._--Shell equivalve, oval, ligament external, cardinal margin straight, toothless, pallial line complete. Palaeozoic----. Principal genus, _Grammysia_, with many other genera of toothless hinge, but whose exact affinities are uncertain.

FAM. 9. _Praecardiidae._--Shell thin, equivalve or not, radiately ribbed, margins dentated, sub-umbonal area as in Arca, hinge toothless. Palaeozoic----. Principal genus, _Praecardium_.

FAM. 10. _Pholadomyidae._--A fourth pallial orifice, siphons very long, united, foot small; shell thin, equivalve, with radiating ribs, ligament external, hinge toothless, pallial line sinuate. Jurassic----. Single genus, _Pholadomya_.

FAM. 11. _Clavagellidae._--Foot rudimentary, siphons long, united, contained in a long calcareous tube; shell small, one or both valves soldered in the tube, tube with a centrally fissured disc at the anterior end, more or less frilled at the posterior end. Cretaceous----. Genera: _Clavagella_, _Brechites_ (= _Aspergillum_).

=Order V. Septibranchiata=

Mantle edges united at three points, branchiae replaced by a muscular septum extending from the anterior adductor to the point of separation of the siphons, septum with symmetrical orifices.

FAM. 1. _Poromyidae._--Branchial septum with groups of lamellae between the orifices, labial palps large, foot long and narrow, siphons short, papillose, separated, animal hermaphrodite; shell small, slightly inequivalve, rounded or oblong, nacreous within. Eocene----. Genera: _Poromya_, _Silenia_.

FAM. 2. _Cuspidariidae._--Siphons long, united in part, foot short, animal dioecious; shell small, slightly inequivalve, rostrate, not nacreous, each valve with ligamentary cartilage spoon-shaped, with a calcareous ossicle, cardinal and lateral teeth present or absent. Jurassic----. Single genus, _Cuspidaria_, with many sections.

BRACHIOPODA

## PART I

RECENT BRACHIOPODA

BY

ARTHUR E. SHIPLEY, M.A.

Fellow and Tutor of Christ’s College, Cambridge

##