16 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



slightly in from the edges, somewhat crenate near the hinge ; both valves 

 distinctly auriculate on the left side near the beaks ; surface with small con- 

 centric marks of growth. Under valve convex ; umbo more or less distorted 

 by the scar of attachment, pointed and distinctly curved to the left. Upper 

 valve nearly Hat, excepting the marginal undulations; beak small, its imme- 

 diate apex curved to the left, but not projecting beyond the cardinal margin. 



Length, from the umbo to the opposite extremity, 1.20 inches; breadth, 

 exclusive of the auricular appendage, 0.70 inch; convexity, 0.45 inch. 



This species is nearly of the same size as the last, and resembles it in 

 some respects; but, so far as yet known, it seems to be distinguished by the 

 decided lateral curvature of its beaks, and the presence of an # auricular 

 appendage on the left side of both valves, near the beaks. 



It appears to be nearly related to O. lateralis, Neilson (=: Gryphma 

 vomer [?], Morton, Synop. Org. Rem., p. 54, pi. 9, fig. 5). It differs from 

 Neilson's species, however, in having its upper valve auriculate, as well as 

 the lower, and its margins subplicate. Its upper valve also differs in not 

 having distinct, regularly-arranged, concentric, imbricating laminae. Both of 

 these species seem to stand, as it were, intermediate between the true 

 oysters and Exogyra. 



Locality and position. — Moreau River; in the Fox Hills group, or forma- 

 tion No. 5 of the Cretaceous series. 



Ostrcit ( O r y p li ee a ! ) patina, M. & H. 

 Plate 10, ti^'s. -J, (i, 6; «, b (bis), and H, c,f; also pi. 11, varieties. 

 Ostrea patina, Meek and Hayden (Nov., 1856), Proceed. Acad. Nat. Sci. Philad., 277. 



Shell subcircular or subovate, a little oblique, rather compressed. Supe- 

 rior valve concave above, thin, excepting at the beak, where it is thickened 

 and truncate ; area broad, triangular, standing nearly at right angles to the 

 plane of the shell, and broadly depressed in the middle; lateral margins near 

 the area more or less contracted. Inferior valve thicker, moderately convex ; 

 lateral margins near the beak sometimes considerably thickened and distinctly 

 lamello-striate ; beak triangular, much compressed, projecting beyond the 

 cardinal border and slightly curved upward at the point, without any visible 

 scar of attachment; area comparatively small, broad, triangular, having a 

 wide, shallow depression along the middle. Muscular impression subcir- 

 cular or transversely oval, moderately distinct, located nearer the left side, 

 and about half-way between the beaks and the postero-ventral extremity. 



