MICHIGAN ACADEMY OF SCIENCE. 



135 



5. Shell, subdiscoidal, aperture circular or nearly 

 so Valvata. 



V. tricarinata. (Say) X2. 



Shell subglobose, aperture not 



circular Somatogyrus. 



- ,,. m S. subglobosus (Say) X3. 



6. i^ielf^^h broadly reflected peristome 7 



Perish ine not reflected, edge of aperture sharp 9 



7. Shell large and dull, more than 5 mm. in 



diameter Polygyra. B 



Shell small and shining, less than 3 mm. in - 

 diameter 8 



p. albolabris (Say) XI. 



8. Lamellae on parietal wall and base of last 



whorl ,^. Strobilops. 



S. labyrinthica rSay) xiO. 



No lamella^' or teeth on inside of last 



v.^horl VaUonia . , . . .^ 



V. piilchella (Mull.) XlO. 



9. Shell dull, coarsely ribbed Pyramidula. ^S^Il^^^ 



Shell shining, usually smooth; a few small northern ^^^^SB^ 

 species minutely ribbed 10 ^^m^ 



- ' p. alternata (Say) XI. 



10. Shell smooth and shining but not vitreous; um- ^ % 



bilicus very wide and deep Circinaria. m' ^ :"-fi'^m><-M 



Shell smooth and shining and vitreous, umbilicus ^ft*^fc«^ 

 narrower , 11 



■' . C. concava (Say) XI. 



11. Shells with' diameter greater than 11 mm., with one exception 



that has 7 to 8 whorls and a diameter of 8 mm 12 



Shells with diameter less than 6 mm., with one exception that 



has 5 to 6 whorls and a diameter of 1\ mm 13 



(The shell mentioned as the first exception in the above is Gastrodo7ita 

 demissa, and may be further distinguished from the other, Zonitoides 

 nitida, by the presence of two lamellae on the peristome of the 

 former. The latter may be told from all the others of No. 11 in that 

 the animal is jet black and the .shell is transparent, so that the 

 living shell appears black). 



