Occasional Papers of the Museum of Zoology 3 



measure 2.75 x 5 mm. This would seem to show the rate of 

 growth. 



The series of this species (Chippewa County, 11; Alger 

 County, 8; Schoolcraft County, 2) is too small to give much 

 idea as to what is really the prevailing type of this region, 

 especially as the series is extremely variable. Thus the two 

 specimens from Schoolcraft County measure : 



Alt. 16.75 m"i- ; Diam. 26.75 mm. ; Axial Index 63. 

 1900 27.50 69. 



The first is a good representative of the fiat northern form, 

 while the latter is quite typical of the southern race. 



The eight specimens from Alger County vary from 25.5 

 mm. to 29.75 mm. in diameter, six of them being between 27 

 mm. and 27.75 mm. in diameter; the altitude varies from 17 

 mm. to 18.75 n^ni- and the axial index from 64 to 69. 



The eleven specimens from Chippewa County vary from 

 25.5 mm. to 29.5 mm. in diameter, and from 16 mm. to 19.5 

 mm. in altitude, and the axial index from 63 to 70 mm., 

 and they exhibit the same tendency toward the northern form 

 on the one hand and the southern form on the other. 



The average dimensions of the three sets are as follows : 



Alger Co., Alt. 17.88 mm., Diam. 27.34 mm... Axial Index 67 



Chippewa Co. 18.36 27.02 68 



Schoolcraft Co. 17-88 27.13 66 



A comparison of the average shell of the whole series with 

 the average shell from the Upper and Lower Peninsula 

 (Walker, 1900, p. 36) and from the Charity Islands (Walker, 

 1915, p. 2) may be made as follows: 

 Up. Pen. Alt. 17.02 mm., Diam. 25.81 mm., Axial Index 65 



^7 

 70 

 68 



