112 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- 
pelago. Three species. 
Melanopsis costata. Melanopsis levigata. 
M., prerosa. : 
FAMILY X. 
Peristomiana. Three genera. 
1. Genus Valvata. Pl. X. 
Animal. See Helix. 
Shell. Found only in fresh water; 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. Three species. 
Valvata piscinalis. 
V. arenifera.* (Leda.) V. tricarinata.* 
2. Genus Paludina. Pl. 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. ‘Thirty-one species. 
Paludina vivipara. Paludina achatina. 
P. Bengalensis. P. Australis. 
P. viridis. P. impura. 
P,. Francesil. P. muriatica. 
