50 CONCHOLOGY. 



C. Fluviatiles. Three genera. 



1. Cyclas. Eleven species. 



The shells of this genus are very small, and are found 

 buried in the mud of fresh waters ; the apices or summits 

 are never eroded, and some species are so thin as to be trans- 

 parent. 



Shell covered with a brown epidermis, oval or sub-orbic- 

 ular, regular, equivalve, inequilateral ; summits blunt, con- 

 tiguous, or turned anteriorly ; hinge similar, complex, formed 

 by a variable number of cardinal teeth, and by two remote 

 lateral teeth with a cavity at the base ; ligament exterior, 

 posterior, and convex ; two distant muscular impressions, 

 without posterior sinus. 



Cyclas rivicola. Cyclas obtusalis. 



C. cornea. C. fontinalis. 



C. lacustris, C. Australis. 



C. obliqua. C. sulcata. 



C. calyculata. C. striatina. 



C. Sarratogea. 



C. cornea. The horny Cyclas. PI. 9, fig. 7. 



Species sub-orbicular, convex, thin, pellucid, with fine 

 concentric striae ; cardinal teeth a little variable, always very 

 small, and sometimes obsolete ; summits not eroded, covered 

 with a horn-coloured epidermis ; interior bluish white. 



2. Cyrena. Ten species. 



This genus of shells is found in rapid rivers and streams ; 

 it was formerly classed with the Cyclas, from which, how- 

 ever, it greatly differs in having three cardinal teeth on each 

 valve, and also lateral teeth. They are thick, solid shells, 

 sometimes of a large size ; the apices always eroded or 

 carious. 



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, placed 



