1910.] NATURAL SCIENCES OF PHILADELPHIA. 27 



The average shell is 22.7 x 15.6 mm., with an index of .68. Com- 

 pared with the remainder of the series, the type is more depressed, 

 having an index of .59 as against an average index of .70. 



2. Var. minor Sterki : 



Alt 15.75 13.75 13.25 



Diam 24.00 20.50 19.25 



Index 66 .67 .69 



The average shell Is 21.25 x 14.25 mm. with an index of .67. 



3. Var. traversensis Leach: 



Alt 14.00 13.50 13.75 13.00 12.25 



Diam 23.75 21.25 20.50 21.00 19.75 



Index 59 .61 .67 .62 .67 



Alt 15.00 13.75 12.50 12.00 



Diam 23.00 22.00 20.50 20.00 



Index .65 .63 .61 .60 



The average shell is 21.31 x 13.31 mm. with an index of .63. At 

 the same time that Dr. Leach collected these recent shells, he also 

 collected a series of fossil shells from the marl lying beneath the present 

 forest, where the typical set was found. The series is of interest as 

 showing that, as a local form, this variety has existed in the same 

 place for a very long period of time and substantially unchanged in 

 character. The dimensions of this series (14) are as follows: 



Alt 15.00 17.75 



Diam 23.00 25.75 



Index 65 .67 



Alt 16.00 14.50 



Diam 25.00 22.00 



Index 64 .66 



Alt 14.75 



Diam 22.50 



Index 66 



The average shell is 22.14 x 14.71 mm. with an index of .66. Com- 

 paring this series with the recent shells, it is to be noted that while 

 the range of variation both in height and width is greater in the fossil 

 series, the average shell in each series is very nearly the same. The 



