58 CONCHOLOGY. 



mud of fresh waters ; summits never eroded, but some species 

 are so thin as to be transparent. 



Shell. With an epidermis, oval, or sub-orbicular, regular, equi- 

 valved, 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 abdomi- 

 nal band, and without posterior excavation. Fifteen species. 



Cyclas cornea. Cyclas rivicola. 



C. obliqua. C. lacustris. 



C. obtusalis. C. calyculata. 



C Australis. C. fontinalis. 



C. striatina. C. sulcata. 



C. Sarratogea.* C. similis.* 



C. Rhomboidea.* C. Partumeia.* 



C. dubia.* 



6. Genus Cyre7ia. PI. 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. Cyrena trigonella. 



C. depressa. C. cor. 



C. fuscata. C. fluminea. 



C. violacea. C. Bengalensis. 



C. Ceylonica. 



C. Caroliniensis.* 



7. Genus Galathea. PI. VII. 

 Animal. As above. 

 Shell. Equivalve, subtrigonal, covered with a greenish epi- 



