1910.] 



NATURAL SCIENCES OF PHILADELPHIA. 



31 



bly the result of some peculiar local conditions. Unfortunately all 

 data on this point are lacking. 



On the whole, therefore, there seems to be reasonable ground for 

 dividing the Michigan series into two groups at this point (24.5 mm.), 

 and for the purposes of this paper they will be treated on that basis, 

 although there is no doubt but that a certain proportion of the series 

 of smaller shells are individual and not racial diminutives. For 

 purposes of convenience and ignoring all questions of synonomy 

 between the varietal names of maritima, minor and traversensis, the 

 smaller race will be termed the "minor" form and the larger race 

 will be referred to as the "major" form. 



JS 60 6i 



Fig. 6- 



6« 6S 68 70 72 T* 70 7g/»«*. 



Axial index — 225 Michigan. 



The range of variation in the axial index of the Michigan series, 

 taken as a whole, is very considerable, ranging from .59 to .75, with 

 an average of .67. The curve is multimodal, but within rather narrow 

 limits, the major mode being at .65, a conspicuous minor mode at .67, 

 and two others of the same height at .63 and .71. Compared with 

 the Cincinnati series, it shows much greater variability but with nearly 

 the same average proportion. One hundred and seven specimens, or 

 48 per cent., are between .64 and .68, while only 63, or 28 per cent., 

 are between .66 and .68, as against 59 per cent, in the Cincinnati 

 series. 



