188 PLANORBIS. 



Genus PLANORBIS, Guettard. 



Besides the typical group, the following 

 well-founded subgenera have been adopted : 



Planorbella, Haldeman. Shell with the 

 whorls few ; aperture expanded, campanulate 

 or bell-shaped, prominent. 



Helisoma, Swainson. Shell ventricose, the 

 spire sunk below the body whorl ; whorls few, 

 often angulated. 



Adula, H. Adams. Shell with the whorls 

 rounded and very numerous, deeply umbili- 

 cated on the upper, and convex on the under 

 side ; aperture campanulate. 



Menetus, H. and A. Adams. Shell de- 

 pressed ; whorls rapidly increasing, periphery 

 angulated. 



Nautilina, Stein. Shell small, orbicular 

 above, flat beneath ; whorls few, rapidly in- 

 creasing. 



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