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. 
Sus-OrpER 2. SUBMYTILACEA. 
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. 
Famity 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—VDreissensia, van Beneden ; an in- 
habitant of fresh water, but originated from the Caspian Sea ; acclimatised 
in England about 1824. Faminry 2. MopIoLARcIDAE, 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). Famity 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. Faminy 4. CRASSATELLIDAE, 
Gray. Mantle with a single suture; foot short. Shell thick with 
concentric striae ; the ligament external. Genera—Orassatella, Lamarck. 
Cuna, Hedley. Faminy 5. CarpiTipa®, Férussac. 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. Faminty 6. CONDYLOCARDIIDAEK, Bernard. Dis- 
tinguished from the family Carditidae by the presence of an external 
ligament. Genera—Condylocardia, Bernard. Carditella, Smith. Cardi- 
topsis, Smith. Faminy 7. Cyprinipag, @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. Plewrophorus, King; fossil from the 
Devonian to the Trias.  Anisocardia, Munier-Chalmas; fossil from 
the Jurassic to the Tertiary. Veniella, Stoliczka; fossil from the Cre- 
taceous to the Tertiary. Famity 8. IsocarpmIpDAE, 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—lIsocardia, Lamarck ; British. 
Faminy 9. CaLLocARDUDAE, 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- 
