286 PALEONTOLOGY. 



I'm IS 



larger sliell, and possesses all the features of a true Ostrca. The fori 

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

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

 vahe 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. Engdmanni Meek, but there 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 Raw- 

 lings Station, Wyoming. 



Genus GRYPH.^A Lam. 



GRYPHiEA CALCEOLA var. NeBRASCENSIS. 

 Plate VII, fi;;. 11. 

 Oryphcea calceola var. Ncbrascemis M. & H., Proc. Acad. Nat. Sei. Pbila., 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 

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

 specimens are mostly small and of variable form, the prevailing 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 abruptl}- 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, 



