26 



PROCEEDINGS OF THE ACADEMY OF 



[Jan., 



The variation in width is within rather larger limits, ranging from 

 25.75 to 32.5 mm., with an average of 28.35. The curve is practically 

 trimodal, the major mode being at 28 and the minors at 27 and 29. 

 Ninety-four specimens or nearly 62 per cent, are from 27 to 29 mm. in 

 diameter. It is also to be noted that only a single example is less than 

 26 mm. in diameter. 



60 61 64 66 68 70 72 Ht Hv»V 



Fig. i — Axial index — 152 Cincinnati. 



In comparison with the variability in height and width, the curve 

 of the axial index is remarkably simple, showing great uniformity in 

 the series in the general proportions of the shell. The curve is practi- 

 cally unimodal, culminating at 67 per cent., the average being 66 per 

 cent. Eighty-nine specimens or 58 per cent, have an index between 

 66 and 68 per cent. 



The Minor Varieties. 



The number of authentic specimens of the three smaller varieties 

 available for study is too small to justify platting a curve for them 

 separately. 



The dimensions of the several specimens of each form are as follows : 



1. Var. maritima Pils.: 



Alt 13. 00 6 17.50 15.25 15.25 16.00 17.00 15.00 



Diam 22.00 24.00 23.00 22.00 22.50 24.00 21.50 



Index 59 .73 .66 .69 .71 .71 .70 



6 Pilsbry's type. 



