84. Whorls with a sharp central keel; western states 



Pyrgulopsis nevadensis (Stearns) 

 Whorls angulated above; southern states 85. 



85. Shell rather slender; no umbilicus; often with spines along the shouldered 



area of the whorls 



Lyrodes coronatus (Pfeiffer) 

 (Potamopyrgus coronatus (Pfeiffer) ) 

 Shell stouter; with a narrow umbilicus; whorls smoothly shouldered 

 Littoridina monroensis (Frauenfeld) 



86. Usually with a small umbilicus; part of operculum with fine, screen'like 



markings 87. 



Shell scarcely or not perforate; no fine, criss-cross lines on the operculum 



91. 



87. Animal amphibious, found on pond edges; shell about two times as high 



as wide, with somewhat turreted whorls; foot with a transverse groove 

 anteriorly 



Pomatiopsis lapidaria (Say) Amphibious Snail 

 Animal aquatic; shell globose to narrow, with convex whorls; foot entire 

 — Lake Shells 88. 



88. Apex flattened, the second whorl surrounding the tiny nuclear whorl 



in the same plane 89. 



Apex acute 90. 



89. With the first three whorls coiled in one plane 



Amnicola emarginata (Kiister) 

 (Cincinnatia emarginata (Kiister)) 

 With the first two whorls coiled in one plane 

 Amnicola limosa (Say) 



90. Spire only slightly longer than the aperture 



Amnicola integer (Say) 

 (Cincinnatia cincinnaUensis (Anthony) ) 

 Spire decidedly longer than the aperture 

 Amnicola lustrica Pilsbry 



91. Lip not continuous 



Hoyia sheldoni (Pilsbry) 

 Lip continuous — • Watercress Snails 92. 



92. Aperture broadly oval, scarcely narrowed above 



Paludestrina nic\liniana (Lea) 

 (Bythinella nic\liniana (Lea) ) 

 (Stimpsonia nicl{liniana (Lea) ) 

 Aperture angled above 



Paludestrina longinqua (Gould) 



93. Shell absent or very small in proportion to the body of the animal, and 



scarcely or not coiled — Slugs and Slug-like Mollusks 94. 



Shell of two or more coils, almost or entirely enclosing the retracted 

 animal — ■ Land Snails 118. 



94. Shell somewhat exposed 95. 

 Shell hidden or absent 98. 



124 



