34 



PROCEEDINGS OF THE ACADEMY OF 



[Jan., 



in the environmental conditions. Compared with the general Michigan 

 series, coming from a very much greater extent of territory and 

 representing the effect of very diverse conditions of environment, 

 it would naturally be expected, like the Cincinnati series, to show much 

 le.-s variation. This is true so far as the height of the shell is con- 

 cerned. But in width the range of variation is much greater and is 

 more similar to that of the general Michigan series. This is owing 

 to the fact that both the Isle Royale and Michigan series extend 

 below 24.5 and include the minor race, which does not appear in the 

 Cincinnati series. 



Fig. 11— Heights— 124 Isle Royale. 



In height the Isle Royale series varies from 14 to 21.5 mm., with 

 an average of 17. As in the Cincinnati series, the curve shows con- 

 siderable variation, but within narrower limits than in the general 

 Michigan series, the range of variation in both being within lh mm., 

 while in the general Michigan series the range of variation covers 

 12^ mm. The curves of both the Isle Royale and Cincinnati series 

 are very similar, the former, however, showing somewhat great 

 variability and averaging 1.78 mm. lower. Fifty-seven specimens or 

 46 per cent, are between 15.5 and 17 mm. in height, as against 50 

 per cent, between 18 and 19 mm. in the Cincinnati series. 



ZZ * 2,3 J- ZH 



ZJ 



ZS 



Zf ± ■So 



2. K.w*\f- 



Fig. 12— Widths— 124 Isle Royale. 



In width the Isle Royale series varies from 21.75 to 30.75 mm., with 

 an average of 25.86. The curve is very similar to that of the general 

 Michigan series, but within somewhat narrower limits, 21.75-30.75 



