102 CONCHOLOGY. 



2. Genus Physa, PI. X. 

 Animal. See Helix. 



Shell. General sinistral, oval, oblong, or globular, very smooth, 

 spire prominent, aperture oval, contracted posteriorly, right edge 

 sharp, columella twisting obliquely, and enlarging to join itself to 

 the anterior part of the margin, the whorls turning to the left 

 hand gives this shell the appellation of heterostrophe. Inhabits 

 the ditches of America and Europe. Six species. 



Physa fontinalis. Physa hypnorum. 



P. castanea.* P. subopaca. 



P. decisa.* P. aurea.* 



3. Genus Pla^wrbis. PI. X. 

 Animal. See Helix. 



Shell. Discoid, spire depressed, or involuted almost in the 

 same vertical plane, causing the shell to be drepressed on each 

 side, aperture oblong, luniforra, axis remote, margin not reflected; 

 no operculum. Inhabits the lakes of America and Europe. 

 Fourteen species. 



Planorbis cornu-arietis. Planorbis orientalis. 



P. corneus, P. carinatus. 



P. spirorbis. P. lutescens. 



P. hispidus. P. nitidus. 



P. angulatus. P. imbricalus. 



P. deformis. P. contortus. 



P. vortex. P. lens.* 



FAMILY IX. 



Melaniaista. Three genera. 



1. Genus Melania. PI. X, 



Animal. See Helix. 



Shell. Turreted ; aperture entire, spire slightly pointed, mar- 

 gin of the whorls often surmounted by spires, columella smooth 

 and arched, closed by a thin horn-like operculum, this is a flu- 

 viatile shell, often covered by a thick epidermis. Inhabits the 

 rivers of India. Many fine species are found in America. Twenty- 

 nine species. 



