The shells of this genus are very small, found buried in the mud of fresh waters; summits never eroded, but some species are so thin as to be transparent.
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.