28 



Bryant Wai^ker 



Genus BYTHINIA Leach, 1818. 



Shell large for the family, spiral, elevated, subperforate ; aperture ova! ; 

 peristome thin, continuous; lip simple, sharp; operculum calcereous, con- 

 centric. 



Type: Helix tentaculata L., fig. 89. Animal, fig. 90. Radula, fig. 91. 



Fig. 89 



Fig. 90 



Fig. 91 



Subfamily AMNICOLIN^ Gill, 1871. 



Shell small, spiral, dextral, subglobose to elongate, thin; imperforate or 

 umbilicate; columella and parietal wall not callously thickened; operculum 

 corneous, paucispiral. 



Foot simple; central tooth of the radula with several denticles. 



Key to genera of Amnicolincc. 



Shell smooth, periphery rounded 2. 



Shell smooth, periphery angulate Littoridina. 



Shell strongly carinated ■ Pyrgnlopsis. 



Shell spinose Potamopyrgus. 



Shell longitudinally ribbed Tryonia. 



Shell slender, spire long , Paludestrina. 



Shell ventricose, spire usually short Amnicola. 



Genus AMNICOLA Gould and Haldeman, 1841. 



Shell small, oval-conic, rather short, spire subacute ; whorls 4-6, convex ; 

 aperture oval; peritreme continuous; lip simple, sharp; columella not thick- 

 ened. Operculum thin, corneous, paucispiral. 



Fig. 92 



Fig- 93 



Fig. 94 



Animal oviparous ; central tooth of the radula multicuspid, with a tooth- 

 shaped process from the middle of the anterior surface, reaching beyond the 



