21. Shell much flattened, with the body whorl distinctly keeled 22. 

 Shell not so much flattened; body whorl sometimes shouldered 23. 



22. Body whorl almost concealing the other whorls, when viewed from 



below; Pacific states 



iVlenetus opercularis (Gould) 

 (Planorhis opercularis Gould) 

 Whorls visible from both sides; widely distributed 

 Promenetus exacuous (Say) 



23. Body whorl shouldered or faintly keeled above; umbilicus small and 



deep, with the body whorl almost concealing the other whorls; eastern 

 and central states 



Menetus dUatatiis (Gould) 

 Body whorl more symmetrically rounded; umbilicus wide, with most 

 of the whorls plainly visible; generally distributed 24. 



24. Shell with cross striations 



Armigera cnsta (Linn.) 

 (Planorbis cristus (Linn.) ) 

 Shell finely striate or smooth 25. 



25. Epidermis erect between the growth lines; aperture oblique 



Gyraulus hirsutus (Gould) 

 (Planorhis hirsutus Gould) 

 Surface smooth; aperture more even 



Gyraulus (Torqws) parvus (Say) 

 (Planorhis parvus Say) 



26. Shell narrow; width less than one-half length 



Aplexa hypnorum (Linn.) 

 Shell wider ' 27. 



27. Shell lengthwise ribbed 



Costatella costata (Newcomb) 

 Shell without lengthwise markings except growth lines 28. 



28. Whorls blending, so that the sutures appear as scarcely more than fine 



lines; spire usually so low that it scarcely disturbs the outline of the 

 body whorl; body whorl usually distinctly shouldered; shell usually 

 with fine spiral lines; Maine to Minn, and south to the Ohio River; a 

 river species 



Physa ancillaria Say 

 Sutures evident; spire usually somewhat acute; body whorl less shoul- 

 dered; shell with or without fine spiral lines 29. 



29. With a thick white callus on the parietal wall and around the edge of 



the lip; seldom over ^2'' high; Great Lakes area to the Gulf 

 Physa integra (Haldeman) 

 Callus around lip purplish or less conspicuous 30. 



30. Shell smooth and shining; seldom much over 1/2'' high; New England; 



rarer west of the Alleghenics; not south of the Ohio and Potomac 

 rivers; small stream form 



Physa heterostropha (Say) 

 Shell with fine spiral lines; becoming larger ' 3L 



117 



