12 Bryant Wai^ker 



Subgenus PLANORBELLA Haldeman, 1842. 



Shell sinistral, depressed, whorls more numerous than in Helisoma; apex 

 scarcely, if any, depressed below the level of the body-whorl; base funicu- 

 lar; body- whorl constricted behind the widely expanded, campanulate 

 aperture. 



Type : P. campanulatiis Say, fig. 27. Radula, fig. 28. 



Fig. 27 



Fig. 28 



Fig. 29 



Subgenus TROPIDISCUS Stein, 1850. 



Shell dextral, moderately large, depressed, upper surface 

 convex, base flattened, adult periphery angular or carinate, 

 the aperture oblique, slightly expanded, simple. 



Type : P. timbilicatus Miiller. 

 Example: P. cultratus d'Orb., fig. 29. 



Subgenus HIPPEUTIS Agassiz, 1837. 



Shell dextral, small, lenticular, with a small number of rapidly increas- 

 ing whorls, the last enveloping a large part of the preceding whorl; apex 

 slightly depressed, base with a narrow umbiHcus, aperture oblique with a 

 thin sharp margin. 



Type : Helix fontanus Lightf oot. 



Section MENETUS H. and A. Adams, 1855. 



Like Hippentis, but the last whorl not enveloping the 

 preceding whorls to so large an extent. 



Type : P. opercularis Gld., fig. 30. 



Subgenus GYRAULUS Agassiz, 1837. 



Shell dextral, small, with few, rapidly increasing whorls, 

 fully exposed above and below, with a nearly median peri- 

 phery, rounded or obtusely angulated, but not accurately car- 

 inated. 



Fig. 30 



