Fam. 4. Ctenodontidae.—Extinct; Silurian.
The fossil group Palaeoconcha is connected with the Protobranchia through the Solenomyidae. It contains the following extinct families.
Fam. 1. Praecardiidae.—Shell equivalve with hinge dentition as in Arca. Praecardium; Silurian and Devonian.
Fam. 2. Antipleuridae.—Shell inequivalve. Antipleura; Silurian.
Fam. 3. Cardiolidae.—Shell equivalve and ventricose; hinge without teeth. Cardiola; Silurian and Devonian.
Fam. 4. Grammysiidae.—Shell thin, equivalve, oval or elongate; hinge without teeth. Grammysia; Silurian and Devonian. Protomya; Devonian. Cardiomorpha; Silurian to Carboniferous.
Fam. 5. Vlastidae.—Shell very inequivalve; hinge without teeth. Vlasta; Silurian.
Fam. 6. Solenopsidae.—Shell equivalve, greatly elongated, umbones very far forward. Solenopsis; Devonian to Trias.
Order II. Filibranchia
Gill-filament ventrally directed and reflected, connected by ciliated junctions. Foot generally provided with a highly developed byssogenous apparatus.
Sub-order I.—Anomiacea.
Very asymmetrical, with a single large posterior adductor. The heart is not contained in the pericardium, lies dorsad of the rectum and gives off a single aorta anteriorly. The reflected borders of the inner gill-plates of either side are fused together in the middle line. The gonads open into the kidneys and the right gonad extends into the mantle. Shell thin; animal fixed.
Fam. 1. Anomiidae.—Foot small; inferior (right) valve of adult perforated to allow passage of the byssus. Anomia; byssus large and calcified; British. Placuna; byssus atrophied in adult. Hypotrema. Carolia. Ephippium. Placunanomia.
Sub-order II.—Arcacea.
Symmetrical; mantle open throughout its extent; generally with well developed anterior and posterior adductors. The heart lies in the pericardium and gives off two aortae. Gills without interlamellar junctions. Renal and genital apertures separate.
Fam. 1. Arcidae.—Borders of the mantle bear compound pallial eyes. The labial palps are direct continuations of the lips. Hinge pliodont, that is to say, it has numerous teeth on either side of the umbones and the teeth are perpendicular to the edge. Arca; foot byssiferous; British. Pectunculus; foot without byssus; British. Scaphula; freshwater; India. Argina. Bathyarca. Barbatia. Senilia. Anadara. Adacnarca.
Fam. 2. Parallelodontidae.—Shell as in Arca, but the posterior hinge teeth elongated and parallel to the cardinal border. Cucullaea; recent and fossil from the Jurassic. All the other genera are fossil: Parallelodon; Devonian to Tertiary. Carbonaria; Carboniferous, &c.
Fam. 3. Limopsidae.—Shell orbicular, hinge curved, ligament longer transversely than antero-posteriorly; foot elongate, pointed anteriorly and posteriorly. Limopsis. Trinacria; Tertiary.
Fam. 4. Philobryidae.—Shell thin, very inequilateral, anterior part atrophied, umbones projecting. Philobrya.
Fam. 5. Cyrtodontidae.—Extinct; shell equivalve and inequilateral, short, convex. Cyrtodonta; Silurian and Devonian. Cypricardites, Silurian. Vanuxemia; Silurian.
Fam. 6. Trigoniidae.—Shell thick; foot elongated, pointed in front and behind, ventral border sharp; byssus absent. Trigonia; shell sub-triangular, umbones directed backwards. This genus was very abundant in the Secondary epoch, especially in Jurassic seas. There are six living species, all in Australian seas. Living specimens were first discovered in 1827. Schizodus; Permian. Myophoria; Trias.
Fam. 7. Lyrodesmidae.—Extinct; shell inequilateral, posterior side shorter; hinge short, teeth in form of a fan. Lyrodesma; Silurian.
Sub-order III.—Mytilacea.
Symmetrical, the anterior adductor small or absent. Heart gives off only an anterior aorta. Surface of gills smooth, gill-filaments all similar, with interlamellar junctions. Gonads generally extend into mantle and open at sides of kidneys. Foot linguiform and byssiferous.
Fam. 1. Mytilidae.—Shell inequilateral, anterior end short; hinge without teeth; ligament external. Mantle has a posterior suture. Cephalic eyes present. Mytilus; British. Modiola; British. Lithodomus. Modiolaria; British. Crenella. Stavelia. Dacrydium. Myrina. Idas. Septifer.
Fam. 2. Modiolopsidae.—Extinct; Silurian to Cretaceous; adductor muscles sub-equal. Modiolopsis.—Modiomorpha. Myoconcha.
Fam. 3. Pernidae.—Shell very inequilateral; ligament subdivided; mantle open throughout; anterior adductor absent. Perna. Crenatula; inhabits sponges. Bakewellia. Gervilleia; Trias to Eocene. Odontoperna; Trias. Inoceramus; Jurassic to Cretaceous.
Sub-order IV.—Pectinacea.
Monomyarian, with open mantle. Gills folded and the filaments at summits and bases of the folds are different from the others. Gonads contained in the visceral mass and generally open into renal cavities. Foot usually rudimentary.
Fam. 1. Vulsellidae.—Shell high; hinge toothless; foot without byssus. Vulsella.
Fam. 2. Aviculidae.—Shell very inequilateral; cardinal border straight with two auriculae, the posterior the longer. Foot with a very stout byssus. Gills fused to the mantle. Avicula; British. Meleagrina. Pearls are obtained from a species of this genus in the Persian Gulf, Indian Ocean, &c. Malleus. Several extinct genera.
Fam. 3. Prasinidae.—Shell inequilateral, with anterior umbones and prominent anterior auricula; cardinal border arched. Prasina.
Fam. 4. Pterineidae.—Extinct; Palaeozoic.
Fam. 5. Lunulicardiidae.—Extinct; Silurian and Devonian.
Fam. 6. Conocardiidae.—Extinct; Silurian to Carboniferous.
Fam. 7. Ambonychiidae.—Extinct; Silurian and Devonian. The last two families are dimyarian, with small anterior adductor.
Fam. 8. Myalinidae.—Extinct; Silurian to Cretaceous; adductors sub-equal.
Fam. 9. Amussiidae.—Shell orbicular, smooth externally with radiating costae internally. Gills without interlamellar junctions. Amussium.
Fam. 10. Spondylidae.—Shell very inequivalve, fixed by the right valve which is the larger. No byssus. Spondylus; shell with spiny ribs, adherent by the spines. Plicatula.
Fam. 11. Pectinidae.—Shell with radiating ribs; dorsal border with two auriculae. Foot byssiferous. Mantle borders with well developed eyes. Pecten; shell orbicular, with equal auriculae; without a byssal sinus; British. Chlamys; anterior auricula the larger and with a byssal sinus; British. Pedum. Hinnites. Pseudamussium. Camptonectes. Hyalopecten; abyssal.
Sub-order V.—Dimyacea.
Dimyarian, with orbicular and almost equilateral shell; adherent; hinge without teeth and ligament internal. Gills with free non-reflected filaments.
Fam. Dimyidae.—Characters of the sub-order. Dimya; recent in abyssal depths and fossil since the Jurassic.
Order III. Eulamellibranchia
Edges of the mantle generally united by one or two sutures. Two adductors usually present. Branchial filaments united by vascular inter-filamentar junctions and vascular interlamellar junctions; the latter contain the afferent vessels. The gonads always have their own proper external apertures.
Sub-order I.—Ostraeacea.
Monomyarian or with a very small anterior adductor. Mantle open; foot rather small; branchiae folded; shell inequivalve.
Fam. 1. Limidae.—Shell with auriculae. Foot digitiform, with byssus. Borders of mantle with long and numerous tentacles. Gills not united with mantle. Lima; members of this genus form a nest by means of the byssus, or swim by clapping the valves of the shell together. Limaea.
Fam. 2. Ostraeidae.—Foot much reduced and without byssus. Heart usually on the ventral side of the rectum. Gills fused to the mantle. Shell irregular, fixed in the young by the left and larger valve. Ostraea; foot absent in the adult; edible and cultivated; some species, as the British O. edulis, are hermaphrodite.
Fam. 3. Eligmidae.—Extinct; Jurassic.
Fam. 4. Pinnidae.—Shell elongated, truncated and gaping posteriorly. Dimyarian, with a very small anterior adductor. Foot with byssus. Pinna; British. Cyrtopinna. Aviculopinna; fossil, Carboniferous and Permian. Pinnigena; Jurassic and Cretaceous. Atrina; fossil and recent, from Carboniferous to present day.
Sub-order II.—Submytilacea.
Mantle only slightly closed; usually there is only a single suture. Siphons absent or very short. Gills smooth. Nearly always dimyarian. Shell equivalve, with an external ligament.
Fam. 1. Dreissensiidae.—Shell elongated; hinge without teeth; summits of valves with an internal septum. Siphons short. Dreissensia; lives in fresh water, but originated from the Caspian Sea; introduced into England about 1824.
Fam. 2. Modiolarcidae.—Foot with a plantar surface; the two branchial plates serve as incubatory pouches. Modiolarca.
Fam. 3. Astartidae.—Shell concentrically striated; foot elongate, without byssus. Astarte; British. Woodia. Opis; Secondary. Prosocoelus; Devonian.
