CONCHOLOGIA CESTRICA. 97 



inhabit still waters, and running streams some pre- 

 ferring the one, some the other. The species are widely 

 distributed, and their appearance so much influenced by 

 locality, as to produce many varieties, which have too 

 often been mistaken for new species, and named as such, 

 causing much confusion, and a needless synonymy. The 

 same remark will apply to many other families of fresh 

 water shells, especially the Strepomatidae. 



The habits of the animals of Pisidium and Sphaerium 

 are somewhat similar, and they are often found together. 

 They are dioicous and viviparous. The breeding season 

 reaches from April to midsummer, during which period 

 they may be found crawling on the surface of the mud. 

 At a later period they bury themselves in the mud, with 

 the aid of the long, extensible foot, where they remain 

 during the ensuing winter. 



S. transversum, SAY. 



Cyclas transversum, Say, New Harm. Dissem., II., 1829. 



SpJweriutn transversum. [Prime.] 

 Fig, 196, 



Shell, transversely oblong, inequilateral, translucent, 

 anterior margin rounded ; posterior subtruncate ; beaks 

 large, calyculate, much elevated ; striae delicate ; green- 

 ish-yellow ; hinge margin nearly straight ; cardinal teeth 

 compressed; laterals slightly elongated. H. n, W. 

 15-16, mill. 



Station, Schuylkill River, Chester County; abundant. 

 9 G 



