_Shell._ With an epidermis, oval, or suborbicular, regular, equivalved, inequilateral; summits obtuse, contiguous or turned anteriorly; hinge similar, complex, formed by a slightly variable number of cardinal teeth, and by two separated lateral teeth with a pit at the base; ligament exterior, posterior, and inflated; two distant muscular impressions, united by a faintly marked abdominal band, and without posterior excavation. Fifteen species. Cyclas cornea. C. obliqua. C. obtusalis. C. Australis. C. striatina. C. rivicola. C. lacustris. C. calyculata. C. fontinalis. C. sulcata. · · · · · C. Sarratogea.* C. Rhomboidea.* C. similis.* C. Partumeia.* C. dubia.* 6. Genus _Cyrena_. Pl. VII. _Animal._ As above. _Shell._ Rounded and trigonal, ventricose, inequilateral; hinge with three teeth on each valve; two lateral teeth, one of which is near the primary ones; ligament exterior, situated on the largest side. The apices are always eroded or carious in shells of this genus. Inhabits the rivers of China. One is found on the coast of Carolina. Ten living species. One fossil. Cyrena orientalis. C. depressa. C. fuscata. C. violacea. C. Ceylonica. C. trigonella. C. cor. C. fluminea. C. Bengalensis. · · · · · C. Caroliniensis.* 7. Genus _Galathea_. Pl. VII. _Animal._ As above. _Shell._ Equivalve, subtrigonal, covered with a greenish epidermis; the surface beneath is white, with several violet streaks radiating from the summit to the margin; two furrowed cardinal teeth upon one valve, three upon the other, the middle one being largest and callous; muscular impressions double and lateral. Inhabits the rivers of Ceylon. One species. Galathea radiata. FAMILY X. CARDIACEA. Five Genera. 1. Genus _Cardium_. Pl. VII. _Animal._ Body somewhat inflated; mantle edged with tentacular cirri in all its inferior part; tubes united, of moderate size, and provided with cirri at the extremity; mouth transverse, very wide, with moderate labial appendages; foot very large, cylindrical, somewhat inclined anteriorly; branchiæ thick, rather small, especially the external laminæ: the internal united in all their extent. _Shell._ Inflated, equivalve, sub-cordiform (when viewed anteriorly, usually costated from the apex to the circumference); summits very evident, but slightly flexed to the front; hinge complex, similar, formed of two oblique, conical cardinal teeth, and of two distant lateral teeth, upon each valve; ligament dorsal, posterior and very short. Inhabits the European and American seas. Fifty-seven living species. Fourteen fossil.