406 Thirty-fifth Report on the State Museum. 



The specimen described has a length of 49 mm., height 32 mm., 

 breadth 30 mm. 



This species differs from 6r. triquetra in its proportionally longer 

 form, less expanded posteriorly ; hinge-line shorter and less oblique to 

 the body of the shell ; and the ventral umbonal slope is less angular 

 along the length. 



Formation and locality. In the Hamilton group, Eighteen-mile 

 creek, Erie county, N. Y. 



