216 Field Columbian Museum — Geology, Vol. i. 



muscular attachment; area small and depressed; muscular impression 

 prominent and possessing fine striae concentrically arranged; dorsal 

 border convex; ventral border nearly straight, somewhat wavy; pos- 

 terior border concave; shell becoming very thin at the margins; exte- 

 rior surface, marked by fine wavy concentric lines of growth which 

 are cut transversely by slight depressions or partings of the shell; 

 interior smooth, corneous. 



Dimensions — Length, 38mm.; width, 42mm.; height, 10mm. 



This species occurs in the Hesperonis beds of the Niobrara. It 

 was collected from the outcrops on the Smoky Hill river in the 

 Kansas area. 



