136 



ELEVENTH REPORT. 



12. Shell subdiscoidal, with very little spire, diameter 

 less than 15 mm Omphalmo. 



Shell subdiscoidal if less than IH mm. in diameter; 

 almost as high as broad if 15 mm. . Gastrodonta . 



G. ligera (Say) Xlo. 



13. Parietal wall and base of last whorl, each with two entering 



lamellae, shell obliquely and coarsely striated ,. . . 8 



No teeth or lamellae with one exception, Vitrea midtiderttata, 

 which has two or more rows of minute teeth, not lamellae, 

 and is not striated 14 



14. Shell subglobose, altitude almost as great as diameter, 



aperture basal Euconulus. 



Shell subdiscoidal, altitude much smaller than 

 diameter; aperture almost lateral 15 



E. fulvus (Mull.).x5. 



15. Shell minute, diameter 1^ mm., with four whorls, 



reddish horn color Punctum. 



Shell with diameter greater than 2^ mm., with two 

 exceptions l)oth of which are pure white and have 

 but three whorls _ 16 



p. pygmaeum (Drap.) X15 



16. Last whorl considerably wider than all of the rest 



combined Vitrina. /'O^ 



Last whorl consideralily narrower than all of the ^^C3 



rest combined 17 



V.limp:da(Goiilil'» X1+. 



17. Shell very transparent, last whorl rapidly increasing 



in diameter towards the aperture, no spiral 

 sculpture Vitrea. 



Shell clouded, whorls increasing gradually in size, 

 shell with microscopic spiral striae. . .Zon?7oi'dcs. 



Z. nitida (Mull.) X2. 



18. 



■'^v'.^""'""/,, 



Aperture without teeth, diameter greater than /^^/^'| 

 10 mm., or shell .sinistral Planorbis. i%\>-^| 



Aperture with teeth, diameter less than 8 mm., 

 shell dextral 19 



p. tiivolvis (Say) XI. 



