216 FIELD COLUMBIAN MusEUM—GEOLOGY, VOL. I. 
muscular attachment; areasmall and depressed; muscular impression 
prominent and possessing fine striz 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, romm. 
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. 
