104 CONCHOLOGY. 



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- 

 pellago. Three species. 



Melanopsis costata. Melanopsis laevigata. 



M. praerosa. 



FAMILY X. 



Peuistomiana. Three genera. 



1. Genus Valvata. PI. X. 



Animal. See Helix. 



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

 bilicated spiral whorls 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, Two species. 



Valvata piscinalis. * Valvata arcnifera. {Lea.) 



2. Genus Paliidina. PI. X. 

 Animal. See Helix. 



Shell. Generally found in fresh water, though some species 

 have been found where it is salt ; conoid covered with 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. Nineteen species. 



Paludina vivipara. Paludina achatina. 



P. Bengalensis. P. Australis. 



P. viridis. P. impura. 



P. Francesii. P. muriatica. 



*P. sinistrosa. *P. hvalina. 



