28 PROCEEDINGS OF THE ACADEMY OF [Jan., 



fossil shell, however, is slightly larger, .S3 mm. in diameter and 1.4 

 mm. in height, and proportionately higher. 



Comparing the average shell of the four series, we have: 



Alt. 



maritima 15.60 



minor 14.25 



traversensis (recent) 13.31 



(fossil) 14.71 



This shows that, while all the western shells are smaller and more 

 depressed than the eastern form, the average minor and fossil travers- 

 ensis, though somewhat smaller than the average maritima, have 

 nearly the same index. The recent traversensis, however, are not 

 only considerably smaller but also proportionately more depressed. 

 While the recent shells are too few in number and too variable to 

 establish a satisfactory curve, the following arrangement of the several 

 frequencies in the 19 specimens is nevertheless of interest, both as 

 showing the range of variation and for comparison with other series. 



Diam 19.25 19.75 20.00 20.50 21.00 21.25 



No 1113 11 



Diam 21.50 22.00 22.50 23.00 23.75 24.00 



No 13 12 13 



Average, 22.97 mm. 



Alt 12.00 12.25 12.50 13.00 13.25 



No 1112 1 



Alt 13.50 13.75 14.00 15.00 15.25 



No 13 12 2 



Alt 15.75 16.00 17.00 17.25 



No 1111 



Average. 14.21 mm. 



Index 59 .60 .61 .62 .63 .65 



No 2 12 111 



Index 66 .67 .69 .70 .71 .73 



No 2 3 2 12 1 



Average, .66. 



Comparing these results with those derived from the Cincinnati 

 series, it is to be noted: 



