1910.] 



NATURAL SCIENCES OF PHILADELPHIA. 



29 



1. That while both series vary greatly in diameter they do not 

 overlap, there being a break of 1.75 mm. between the largest of the 

 "minor" series and the smallest of the Cincinnati shells. The average 

 diameter of the minor series is 22.97, as against 28.35 mm. in the 

 Cincinnati series, and that while 62 per cent, of the Cincinnati series 

 are from 27 to 29 mm. in diameter, more than 68 per cent, of the 

 minor series are from 20 to 23 mm. in width. 



2. That notwithstanding this great difference in size, the propor- 

 tions of the shells of both series are substantially the same, the index 

 of the minor series ranging from .59 to .73 and that of the Cincinnati 

 series from .60 to .74, the average index in both series, however, 

 being the same, .66. 



3. That while perhaps it may be claimed that the minor series is 

 too small, in proportion to the Cincinnati series, to give any satisfactory 

 comparison, nevertheless the evidence, such as it is, certainly tends 

 to show the existence of a smaller race of substantially the same 

 general shape, but averaging 5.36 mm., or 19 per cent., smaller in 

 diameter. 



The General Michigan Series. 

 As shown by the figure, this series exhibits great variability in 



i^iii{*t«iat«i^i^i^t«i4i*t*^*i'"" f—4 



Fig. 5 — Widths — Upper line, 225 Michigan and 124 Isle Royale. Lower line, 



225 Michigan. 



size, ranging from 18.75 to 34.25 mm. in width, with an average of 

 26.95. The major mode is at 29, with minor modes at 30.5, 28, 26.75 

 and 25. The average is 26.95 mm. There is a decided break at 

 24.5, below which there is a series of minor modes at 24.23, 22 and 

 19.75. 



Were it not for the strong minor mode at 24.75 and 25, the break 

 at 24.5 would be much more conspicuous. As the fifteen specimens 



