1910.] 



NATURAL SCIENCES OF PHILADELPHIA. 



37 



The Upper Peninsular series varies from 21.75 to 30.75 mm. with 

 an average of 25.81, while the Lower Peninsular series ranges from 

 18.75 to 34.25 mm. with an average of 27.10. 



It is to be noted that both curves show the same decided break at 

 24.5 mm. 



Forty-seven or 25.7 per cent, of the Lower Peninsular and 44 or 

 26.5 per cent, of the Upper Peninsular shells are 24.5 mm. or less 

 in diameter, while 122 or 73.5 per cent, from the Upper Peninsula 

 and 136 or 74.3 per cent, from the Lower belong to the major race 

 so called. 



SI 6o 6% 64; 66 6S lO 1Z 74 76 IS iwv. 



Fig. 16 — Axial index — a to b, 183 Lower Peninsula; c to d, 166 Upper Peninsula. 



The curves of the axial index of the two series are almost exactly 

 the same, the modes being in every instance at the same point, -but 

 with some slight variation in the number of frequencies and with a 

 slightly larger range of variation in the Lower Peninsular series. The 

 average index in the Upper Peninsular series is .654, as against .677 

 in the Lower Peninsula. 



The average shells of the two series compare as follows : 



Alt. Diam. Index. 



Upper Peninsular 17 02 25.81 .654 



Lower Peninsular 18.10 27.10 .677 



The average Upper Peninsular shell is 25.81 x 17.02 mm. If it 

 were of the same proportions as the average Lower Peninsular shell, 

 it should be 17.24 mm. in height and of course with the same index. 



