286 PALEONTOLOGY. 



larger shell, and possesses all the features of a true Ostrea. The form is 

 irregularly ovate in outline, moderately convex, and slightly curving poste- 

 riorly ; length and breadth about as four to three, the expansion of the 

 valve being most rapid on the posterior side below the middle ; posterior 

 border concave in the upper part, and sharply rounded below ; anterior 

 border regularly and broadly rounded ; adductor muscular scar small, sub- 

 marginal, situated above the middle of the length ; the area embraced 

 above the pallial line being not more than one-fourth as great as that 

 below. 



The specimen under consideration was at first supposed to be the young, 

 or a small individual, of 0. Engelmanni Meek, but thei'e is not the slightest 

 evidence of plications, the shell is proportionally longer, and the muscular 

 imprint proportionally smaller and more nearly submarginal ; yet the 

 resemblance to that species is quite strong, and it is possible that in such 

 variable shells such changes may take place in the same species. 



Formation and locality. — In rocks of Jurassic age, northwest of Kaw- 

 lings Station, Wyoming. 



Genus GRYPH^A Lam. 

 Gryph^a calceola var. Nebrascensis. 



Plate VII, fig. 11. 



Gryphcea calceola var. Nebrascensis M. & BL., Proc. Acad. Nat. Sci. Phila., 1861, p. 437 ; 

 Pal. Upper Missouri, pp. 74-75, pi. 3, fig. 1. 



Among the Jurassic fossils of the collection are numbers of a small 



Oyster-like shell, which we suppose to be identical with many of those 



referred to the above-named variety of Quenstedt's species G. calceola. The 



specimens are mostly small and of variable form, the prevaihng feature 



being broadly and irregularly reniform, or curved-ovate ; more or less 



truncate at the posterior end ; the smaller valve being extremely shallow 



and scarcely convex, while the attached valves are very irregular and 



variable in depth and convexity, most of them being flattened and attached 



over the greater part of their extent, with the edges abruptly curved 



upward, to give the requisite depth, others scarcely showing any mark of 



attachment, and still others are squarely and vertically truncate at the 



upper extremity, similar to those represented in the Pal. Upper Missouri, 



