348 AMPULLARIIDiE. 



culum, M. cornu-arietis, fig. 3, a. 3, h. Shell, M. cornu- 

 arietis, fig. 3, c. 



The shells of this genus are more or less discoidal, 

 and, until the animal was made known by Guilding, were 

 often regarded as species of Planorhis. 



Species oj Marisa. 



castanea, Desk. fasciata, Guild. 



cornu-arietis, Linn. luteostoma, Swains. 



effusa, MiilL ' pachystoma, Phil. 



Genus POMELLA, Gray. 



Operculum horny, dextral. 



Shell solid; spire short, whorls transversely striated, 

 the last very large; aperture semi-ovate; inner lip con- 

 cave, broad, flattened, peritreme simple, acute. 



Ea;. P. megastoma, Sowerhy, pi. 37, fig. 4. Oper- 

 culum, P. megastoma, fig. 4, a, 4, b. 



The animal of this genus, of which but a single species 

 is known, has not hitherto been observed. The shell is 

 transversely grooved, of dense structure, and the colu- 

 mellar lip is peculiarly flattened. It is an inhabitant of 

 the rivers of South America. 



Genus LANISTES, Montfort. 



Operculum horny, sinistral, or with the nucleus on the 

 left margin. 



Shell depressed, thin, sinistral, deeply and widely um- 

 bilicated ; spire short ; aperture oblong, entire ; inner lip 

 expanded over the last whorl, peristome simple, acute. 



