58 CONCHOLOGY. 
mud of fresh watets; 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. 
it C. Sarratogea.* C. similis.* 
C. Rhomboidea.* C. Partumeia.* 
C. dubia.* 
6. Genus Cyrena. 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 en the coast 
of Carolina. ‘Ten living species. One fossil. 
Cyrena orientalis. Cyrena trigonella. 
C. depressa. cor: 
C. fuscata. C. fluminea. 
C. violacea. C. Bengalensis. 
C. Ceylonica. 
©. Caroliniensis.* 
7%. Genus Galathea. Pl. VII. 
Animal. As above. 
Shell. Equivalve, subtrigonal, covered with a greenish epi- 
