DESCRIPTIONS OF NEW SPECIES. 127 



Hptical or subovate in marginal outline ; valves moderately and somewhat 

 regularly convex ; test not massive.; dorsal margin broadly arched ; front 

 margin regularly rounded ; basal margin broadly and regularly rounded ; 

 posterior margin somewhat abruptly rounded, the postero-dorsal portion 

 sometimes obliquely truncated and sometimes sloping to a more prominent 

 posterior extremity; beaks well denned, but not prominent; umbones 

 broadly convex. 



Surface marked by the ordinary lines and lamellations of growth. 



Length of the largest example in the collection, nearly seven centi- 

 meters ; height of the same, five centimeters. 



This species bears some resemblance to U. Haydeni Meek, from the 

 Bridger Group, but differs from that species in its larger size, its convex 

 instead of straight dorsal margin, its rather more prominent umbones, and 

 its greater propor donate height. 



Position and locality. Upper Green River Group; Henry's Fork and 

 Alkali Stage Station, Wyoming ; also in Lower Green River Group ; west 

 side of Snake River, six miles north of Junction Mountain ; Cliffs, four 

 miles northeastward from Vermilion Canon ; and Dry Mountains, North- 

 western Colorado. 



Genus CORBICULA Miihlfeldt. 



Corbicula Powelli (sp. nov.). Shell rather small, subelliptical in mar- 

 ginal outline ; valves thin, slightly but somewhat uniformly convex ; beaks 

 small, not prominent ; cardinal and lateral teeth well developed ; both 

 anterior and posterior lateral teeth finely crenulated transversely ; middle 

 cardinal tooth of each valve having a shallow vertical groove along its 

 middle ; a very faintly-raised ridge extends downward from beneath the 

 beak on the inner surface of each valve, and dies out before reaching 

 the base. Surface nearly smooth, but marked by fine lines of growth. 



Length, twenty-three millimeters ; height, from beak to base, fifteen 

 millimeters ; thickness, eight millimeters. 



This shell differs from typical forms of Corbicula in its elliptical out- 

 line, slight thickness, and in the delicacy of the test. All the species 

 associated with it, except an oyster, are exclusively marine forms. 



