350 GEOLOGY OF THE BLACK HILLS. 



and more or less concave near the middle, usually concave on the inner 

 side toward the cardinal extremity, which is thick and truncate; surface 

 with rather distinct concentric marks of growth." (M. & H.) 



Exterior surface of the larger valve often marked by distinct and some- 

 times wiry, irregular, longitudinal striae, which radiate from the apex and 

 continue to below the middle of the valve. 



Messrs. Meek and Hayden formerly united this and the preceding- 

 species (Ostrea strigilecula), under the same name, as a variety of Quen- 

 stedt's G. calceola. It appears to us, however, from an examination of spec- 

 imens, that they are clearly distinct, generically as well as specifically, and 

 that if the same variations of form occur associated with each other in the 

 European localities, it is very probable that the two genera are also there 

 represented as well as in our American formations. 



Formation and locality. — In rocks of Jurassic age, Wind River Mount- 

 ains. 



PECTENID.E. 



Genus PECTEN Bruguieres. 

 PECTEN NEWBEKKYI. 



Plate 4, figs. 12-15. 



Pecten newberryi Whitf., Prelim. Eept. Pal. Black Hills, 1877, p. 16. 



Shell of moderate size, suborbicular in outline, erect, and subequilat- 

 eral, the height and length nearly equal. Valves depressed-lenticular in a 

 transverse section when united; their surfaces .depressed-convex, except 

 along the sides of the valves near the cardinal slopes, where the surface is 

 elevated so as to form a broad, rounded, fold-like border on this part, with 

 a broad, undefined, shallow depression separating it from the body of the 

 valve. Hinge line short ; that of the left valve less than half as long as the 

 shell below; straight, or a very little sloping outward from the apex of the 

 valve, which is small, appressed, and not projecting above the line of the 

 hinge Anterior side of the hinge a little longer than the other, the ante- 

 rior wing being rounded at the extremity, more than as long again as the 

 height above the body of the valve at its widest part; separated from the 

 shell below by a very shallow, rounded byssal notch, and along the surface 



