MICHIGAN ACADEMY OF SCIENCE. 



137 



19. Aperture with slight inner callus; diameter <i;reater iZ^ 



than 6 mm., color brown Segmentina. g. aimigeia (Say)xi. 



Aperture Avith lamellae that run around the inside 

 of all of the whorls, and which are visible from 

 the outside; diameter less than 4 mm., color 



greenish horn Helicodiscus. 



H. parallelus (Say) X5. 



^^CZ) 



A. hypnorum (L.) XI 



20. Shells sinistral 21 



Shells dextral 22 



21. Shell very long and narrow, aperture attenuated 



above, columellar lip simple, mantle with simple 

 edge Aplexa. 



Shell and aperture nearly oval, columella provided 

 with callus which is spread over last whorl, 

 mantle fringed with finger-like filaments. .P/?i/so. 



p. gyrina (Say) Xl 



22. With operculum, peristome continuous or continued by a callus 



so that the operculum entirely closes the shell, shell heavy 



although often small 23 



Without operculum, peristome not continuous, shell thin and 



fragile 29 



28. Shell large, altitude more than 18 mm 24 



Shell small, altitude not more than 11 mm 25 



24. Shell usually l^anded. aperture and oper- 

 culum almost circular Vivipara. 



V, contectoicles (W. G. B.) XI 



Shell not l)anded, ai)erture narrow-oval, 

 with upj^er side straight Campelonia: ( 



C. decisum (Say) X 1 . 



25. Shell subglobose, aperture two-thirds as high as entire shell 5 

 Shell conical, aperture not one-half as high as entire shell 26 



26. Altitude of shell about 10 mm. ; operculum almost 



circular with a blunt projection pointing up- 

 wards Bythinia. 



Altitude of shell not over 7 mm. ; operculum with- 

 out such a projection 27 



18 



B. teiitaculata (L.) XI 5. 



