1910.] 



NATURAL SCIENCES OF PHILADELPHIA. 



35 



as against 18.75-34.25. The average shell is smaller, being 25.86 

 as against 26.95 nun. 



It is to be noted that a decided break occurs in the curve at 24.5, 

 as in the general Michigan series. Thirty-six specimens or 28 per 

 cent, are 24.5 or less in diameter, as against 24 per cent, in the general 

 series. 



Eliminating these, the curve of the remaining 88 specimens of the 

 major form is quite similar in a general way to that of the Cincinnati 

 series and to that of the major race in the general Michigan series, 

 but is like the latter in showing greater variability as compared with 

 the Cincinnati series. The range of variation in the Isle Royale and 

 Cincinnati series is about the same, but only two-thirds that of the 

 general Michigan series. The three series may be compared as follows : 



Cincinnati varies from 25.75 to 32.5 or within 6.75 mm.; average 



28.35. 

 Michigan varies from 24.75 to 34.25 or within 9.50 mm.; average 



29.31. 

 Isle Royale varies from 24.75 to 30.75 or within 6.00 mm. ; average 



26.97. 



^3 j-6 ■•ys Go 61 sa 66 <*? lo li. Tif THn\. 



Fig. 13— Axial index— 124 Isle Royale. 



The axial index of the Isle Royale series varies from .59 to .72, with 

 an average of .65. The curve is bimodal, with the major mode at .65 

 and the minor at .67. It is intermediate between that of the Cincin- 

 nati series and that of the general Michigan series, which it resembles 

 more than it does the former, differing mainly in being more simple 

 by the suppression of the conspicuous minor modes at .61, .63 and .71 . 



Comparative Notes upon the Different Series. 

 ] . The shells of the Upper and Lower Peninsulas. 



