112 CONCHOLOG\. 



Shell. The Melanopsis is distinguished from the Melanaia, by 

 the columella being callous in the upper part, and the base trun- 

 cated as in the Achatina, and differs from the Pirena in having 

 only a sinus or widened opening at the base of the shell ; aperture 

 covered by a horny operculum. Inhabits the rivers of the Archi- 

 pelago. Three species. 



Melanopsis costata. Melanopsis laevigata. 



M. prsarosa. 



FAMILY X. 



Peristomiana. Three genera. 

 1. Genus Valvata. Pi. X. 

 Animal. See Helix. 



Shell. Found only in fresh v^^ater; subdiscoid or conoid, um- 

 bilicated, spiral, v^'horls, rounded ; angular at the summit ; aperture 

 round, not modified by the penultimate whorl ; the margins sharp 

 and united ; operculum orbicular and horny. Inhabits the rivers 

 of Europe and America. Three species. 

 Valvata piscinalis. 



V. arcnifera.* {Lea.) V. tricarinata.* 



2. Genus Pahidina. PI. X. 

 Animal. See Helix. 



Shell. Generally found in fresh water, though some species 

 have been found where it is salt ; conoid covered v^^ith a greenish 

 epidermis; the whorls rounded or convex, spiral cavity modified 

 by the last whorl ; aperture rounded, oval, oblong, angular at the 

 summit ; margins united, acute, never reflected outwards ; opercu- 

 lum orbicular and horny. Inhabits the rivers of America and 

 Europe. Thirty-one species. 



Paludina vivipara. ll Paludina achatina. 



P. Bengalensis. P. Australis. 



P. viridis. P. impura. 



P. Francesii. P. muriatica. 



