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, anda needless synonymy. The 
same remark will apply to many other families of fresh 
water shells, especially the Strepomatidz. 
The habits of the animals of Pisidium and Spherium 
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., IT., 1829. 
Spherium transversum,—[Prime.] 

Shell, transversely oblong, inequilateral, translucent, 
anterior margin rounded; posterior subtruncate; beaks 
large, calyculate, much elevated; striz delicate; green- 
ish-yellow ; hinge margin nearly straight ; cardinal teeth 
compressed; laterals slightly elongated. H. 11, W. 
15-16, mill. 
Station, Schuylkill River, Chester County ; abundant. 
9 G 
