STANTON.] OSTREID^. G5 



"The collections contain numerous examples of this species, the type 

 specimens of which were obtained by Dr. lloemer from near San Anto- 

 nio, Texas. The figure given by Dr. Koemer (loc. cit.) represents the 

 the umbo of the larger valve more nearly free than that of any of our 

 examples is, the umbo in all of our examples being closely incurved. 

 In this respect ours are more nearly like those figured and described 

 by Conrad. 



"Position and locality. — Strata of the Cretaceous period, Linear pla- 

 teau, southeastern Utah." 



The above description is quoted from Dr. White's report in the Sur- 

 vey West of the one hundredth Meridian. Coquand proposed a new 

 name, for the species because Minister had previously described an 

 Ostrea Iceviuscula, but as Miiuster's species was not an Exogyra the 

 change was unnecessary. 



It may be that Mr. K. T. Hill is right in regarding these fossils as 

 young individuals of Exogyra ponderosa. The fact that they occur 

 together would cause one to suspect their identity, but those described 

 under the name E. Iwviuseula are smoother shells, with a more nearly 

 circular outline and less angular umbonal region than E. ponderosa, 

 even when of the same size. 



Exogyra ponderosa Koemer. 



PL vn, Figs. 1, 2. 



Exogyra ponderosa Koemer, 1852, Kreidebilduugen von Texas, p. 71, PI. 9, Figs. 2a, h; 



White, 1876, U. S. Geog. & Geol. Sur. West 100th Meridian, vol. IV, p. 172, PI. 



14, Figs, la-c; 1884, 4th Ann. Rept. U. S. Geol. Sur., p. 306, PI. 50, Figs. 1, 2, 3. 

 Exogyra costata, var. Con., 1857, U. S. & Mex. Bound. Sur., vol. I, p. 154, PI. 8, Fig. 



3, PL 9, Fig. 1. 

 tExogyra fimbriata Con., 1857, Ibid., PL 7, Figs. 2a, b. 



" Shell large, capacious ; margin al outline irregularly subovate ; larger 

 valve very gibbous; umbo distinctly spiral, but the coil is usually 

 obscured by a large scar of attachment; umbonal half obtusely ('an- 

 nate, the sides sloping abruptly from the carina to the margins; basal 

 half not so deeply, but more regularly convex than the other. Test 

 very massive, sometimes having a solid thickness of live or six centime- 

 ters, lamellose, so much so that the valve often splits into numerous 

 pieces along the surfaces of the layers of growth; inner surface smooth; 

 muscular scar of moderate size, somewhat deep, placed about midlength 

 of the valve, and, as usual, a little nearer to the posterior than to the 

 anterior side; surface marked by strong, irregular, imbricating lamel- 

 lae of growth, which become laciniate at and near the margins; surface 

 also marked by fine concentric striae, and by irregular, indistinct, radi- 

 ating costre, the latter being usually removed by exfoliation from old 

 shells. The collections do not contain any example of the upper valve, 

 but both Koemer and Conrad describe it as thick, concentrically lam- 

 inated; smooth within; umbo horizontal, distinctly spiral. 

 Bull. 106 5 



