264 THE LAMELLIBRANCHIA 



muscle. Shell truncated and gaping posteriorly. Foot byssiferous. 

 Genera Pinna, Linnaeus ; heart traversed by the intestine ; anus pro- 

 jecting and appendiculated. Cyrtopinna, Morch. Aviculopinna, Meek ; 

 fossil from the Carboniferous and Permian. Pinnigena, de Saussure ; 

 fossil from the Jurassic and Cretaceous. Atrina, Gray ; from the Car- 

 boniferous to the present day. 



SUB-ORDER 2. SDBMYTILACEA. 



Eulamellibranchia in which the mantle is only slightly closed ; 

 generally there is only a single suture. Siphons absent or very short. 

 Gills smooth. Nearly always dimyarian. Shell equivalve, with an 

 external ligament. 



FAMILY 1. DREISSENSIIDAE, Gray. Two pallial sutures and two 

 short siphons ; pedal orifice short. Foot cylindrical with a stout byssus. 

 Shell elongated ; the hinge without teeth ; the summits of the valves 

 with an internal septum. Genus JJreissensia, van Beneden ; an in- 

 habitant of fresh water, but originated from the Caspian Sea ; acclimatised 

 in England about 1824. FAMILY 2. MODIOLARCIDAE, Gray. Mantle 

 with two sutures. The foot byssiferous, with a plantar surface and a 

 glandular cavity in front of the byssogenous cavity. The two branchial 

 plates serve as incubatory pouches. Genus Modiolarca, Gray ; sub- 

 antarctic (Fig. 241). FAMILY 3. ASTARTIDAE, d'Orbigny. A single 

 pallial suture. Foot elongate, without a byssus. Shell concentrically 

 striated ; the ligament external. Genera Astarte. Sowerby ; British. 

 Woodia, Deshayes. Opis, Defrance ; fossil from the Secondary. Pro- 

 socoelus, Keferstein ; fossil from the Devonian. FAMILY 4. CRASSATELLIDAE, 

 Gray. Mantle with a single suture ; foot short. Shell thick with 

 concentric striae ; the ligament external. Genera Crassatella, Lamarck. 

 Cuna, Hedley. FAMILY 5. CARDITIDAE, Fe'russac. Mantle with a single 

 pallial suture ; foot carinated, often byssiferous ; palps short. Shell 

 thick with radiating costae ; the ligament external. Genera Cardita, 

 Bruguiere. Thecalia, Adams. Milneria, Dall ; incubatory, California. 

 Venericardia, Lamarck. FAMILY 6. COXDYLOCARDIIDAE, Bernard. Dis- 

 tinguished from the family Carditidae by the presence of an external 

 ligament. Genera Condylocardia, Bernard. Carditella, Smith. Cardi- 

 topsis, Smith. FAMILY 7. CYPRINIDAE, d'Orbigny. Mantle open in 

 front, and with two pallial sutures. The branchial and anal orifices 

 papillose, the latter projecting. External gill -plates smaller than the 

 internal. Genera Cyprina, Lamarck ; British. Cypricardia, Lamarck. 

 Coralliophaga, de Blainville. Pleurophorus, King ; fossil from the 

 Devonian to the Trias. Anisocardia, Munier-Chalmas ; fossil from 

 the Jurassic to the Tertiary. Venidla, Stoliczka ; fossil from the Cre- 

 taceous to the Tertiary. FAMILY 8. ISOCARDIIDAE, Gray. Mantle 

 largely closed, the pedal orifice generally small ; the anal and branchial 

 orifices sessile ; gill-plates of equal size ; foot short. Shell globular with 

 prominent and coiled umbones. Genus Isocardia, Lamarck ; British. 

 FAMILY 9. CALLOCARDIIDAE, Dall. The anal and branchial orifices of 

 the mantle provided with siphons. The external gill-plate smaller than 

 the internal. Shell ventricose, but elongated ; the umbones not promi- 



