510 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



short, and usually curved to the left; the area being small and rather deep. 

 The surface of both valves is entirely free from radiating plications or costas, 

 or projecting concentric laminae ; though it shows obscure striae, and a few 

 irregular, faintly-marked ridges of growth. 



Locality and position. — Judith River beds, at the mouth of Judith River, 

 Montana, on the Upper Missouri ; probably belonging to the closing period 

 of the Cretaceous. 



Ostren stibtrigonalis, E. & S. ? 



Plate 40, figs. 1, a, i, c, d. 

 Ostrca subirigonalis, Evans aud Shumanl (1857), Trans. Acad. Sci. St. Louis, 40. 



Shell small, rather depressed, narrow-subovate in outline, generally 

 slightly arcuate laterally ; beaks rather pointed when not distorted by the 

 scar of attachment, while the opposite extremity is compressed and more or 

 less rounded. Lower valve rather shallow, with lateral margins crenate and 

 sometimes a little furrowed, but not thickened or expanded ; ligament-area 

 moderate, triangular, and with the usually pointed beak turned a little to the 

 left; surface merely showing concentric striae and furrows. Upper valve 

 nearly flat, or much compressed, with beak more truncated than that of the 

 lower; surface much as in the other valve. 



Length of a mature specimen, 1.82 inches; breadth, about 0.87 inch; 

 convexity, about 0.40 inch. 



This is the form that we have always identified with Evans and Shu- 

 mard's O. subtrigonalis ; but as they never figured their species, and I have 

 had no opportunity to compare our specimens with authentic examples of it, 

 the identification cannot be regarded as entirely beyond question. Indeed, it 

 is quite probable that a direct comparison might show our shell to be specific- 

 ally distinct, especially as we have no other evidence that the same forma- 

 tion occurs at the two localities. In case our specimens may be found to 

 belong to a different species, I would propose to call \t.O. Shumardi, in honor 

 of my lamented friend, the late Dr. B. F. Shumard. 



This species will be readily distinguished from the last by its smaller 

 size, narrower and more regular form, shallower lower valve, and its greater 

 thickness in proportion to itssize. 



Locality and position — Same as last. 



