172 UNIVALVE SHELLS 



Animal deep black, immaculate, above and be- 

 neath; tentacula setaceous, a white annulation at 

 base. 



In the fragility of the shell, this species ap- 

 proaches nearest to columella. It is very common 

 in stagnant ponds on the banks of the Mississippi. 

 When the shell includes the animal, it appears of a 

 deep black colour, with an obsolete testaceous spot 

 near the base on the anterior side. Its proportions 

 are somewhat similar to those of P. hypnorum, 



P. heterostropha (Nicholson's Encyc.)Is very com- 

 mon in ponds of the Missouri as far as Council 

 Bluff. 



Genus Cyclostoma. 



C. *marginata. Shell turreted, pale horn colour, 

 or dusky, obsoletely wrinkled across ; suture rathei 

 deeply impressed ; volutions six ; aperture mutic, 

 sub-oval, truncated transversely above by the penul- 

 timate whirls, nearly 1-3 the length of the shell ; 

 labium nearly transverse, colour of the exterior part 

 of the shell ; labrum equally aud widely reflected, 

 thick, white ; umbilicus distinct. 



Inhabits Upper Missouri. 



Length 1-5 of an inch. 



Size of Paludina lapidaria. Lister represents a 

 species on plate 22 fig. 19, which, although rather 

 larger, may possibly be intended for this species ; he 



