Aperture rounded in front, without notch or canal. 



Genus GONIOBASIS 5 

 2 — Whorls more rounded, suture impressed. 



Shell rather thin, whorls very rounded, color usually- 

 dark, often with color bands. Genus VIVIPAEA 6 

 Shell heavy, spire produced, whorls all slightly flattened, 

 color usually light greenish. 



Length more than 15 mm., at least twice the diameter. 



Genus CAMPELOMA 7 

 Length less than 15 mm., less than twice the diameter. 



Genus LIOPLAX S 



b — Shell less than 10 mm. long, 



1 — Spire distinctly longer than the aperture. 

 Whorls flattened or carinate, apex acute. 



Genus PYEGULOPSIS 9 



Whorls rounded, apex obtuse. 



Brownish, 5 mm. long; terrestrial. 



Genus POMATIOPSIS 10 

 Greenish, 2/3 mm. long; aquatic. 



Conical. Genus POMATIOPSIS 10 



Subcylindrical, obtuse. Genus BYTHINELLA 11 



2 — Spire not exceeding aperture. 

 Whorls rounded. 



Shell very oblique. Genus SOMATOGYEUS 12 



Shell more regularly conical. Genus AMNICOLA 13 

 Whorls flat or sharply angular. 



Suture prominent, whorls shouldered, carinate; um- 

 bilicus wide and deep. Genus VALVATA 14 

 Suture scarcely impressed, whorls flattened; umbilicus 

 closed; terrestrial. Genus HELICINA 15 

 B — Not operculate. 

 a — Aquatic. 



1 — Shell conical, limpet-like (not spiral). 

 Apical cap and internal shelf present. 



Genus GUNDLACHIA 16 

 Apical cap and internal shell absent. Genus ANCYLUS 17 



2 — Shell conical, spiral. 



Dextral. Genus LYMN^A 18 



Sinistral. 



Narrow, with exserted spire; polished. 



Genus APLEXA 19 

 Broader, spire shorter; usually less polished. 



Genus PHYSA 20 



3 — Shell flattened, spire not exserted. 



Aperture with teeth or folds. Genus SEGMENTINA 21 

 Aperture unarmed. Genus PLANOEBIS 22 



b — Terrestrial. 



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