CEETACEOUS FOSSILS. 385 



ventricose, projecting considerably above the cardinal border, slightly 

 incurved and directed obliquely forward ; body of the valve highly con- 

 vex and very oblique ; anterior and basal margins sloping backwards, with 

 a shallow sinuosity below the anterior wing ; posterior extremity obliquely 

 rounded; ligamental area proportionally large and slightly concave. Right 

 valve smaller and much less convex than the left ; beak small, pointed, 

 directed obliquely forward and not extending above the cardinal line; 

 anterior wing apparently larger than on the left valve, much more convex 

 and pointed ; posterior wing of the same form as that of the left valve, but 

 concave toward the upper edge. Muscular imprint large, subcentral, a 

 little nearer the posterior margin and distinctly subreniform ; pallial line 

 composed of elongated pustules. Substance of the inner layers of the 

 shell nacreous ; outer layers, as far as ascertained, very finely prismatic. 



This species appears to have been a common form in the seas of this 

 region during the deposition of the rocks of the Fort Pierre - Group, as the 

 broken and comminuted shells are quite numerous in the dark concre- 

 tionary layers of this formation at most of the localities from which col- 

 lections have been made, though but few individuals are obtained in a good 

 state of preservation. It is an easily recognized form. 



Formation and locality. — In limestones of the Fort Pierre Group, on 

 the South Fork of the Cheyenne River, near French Creek, and on Old 

 Woman Fork, Black Hills. 



PTEBI& (OXYTOMA) NEBBASCANA. 



Plate 7, fig. 4. 



Avicula nebrascana E. & Skumard, Trans. St. Louis Acad. Sci., vol. i, p. 38. 

 Pteria nebrascana (E. & S.) Meek, Sinitksonian Inst. Okeck List Inv. Eoss., p. 9. 

 Pteria (Oxytoma) nebrascana (E. &. S.) Meek, Pal. U. S. Geol. Surv. Terr., p. 34, PI. 16, 

 fig. 3, and pi. 28, fig. 11. 



Shell of moderate size and smaller, obliquely rhombic-ovate in form, 

 very inequilateral. Left valve depressed-convex, with a small, pointed, and 

 suberect beak, slightly extending above the cardinal line and situated 

 about one-fourth of the entire length from the anterior end ; hinge line two- 

 thirds as long as the shell below ; anterior wing very small, obtusely 

 rounded ; anterior margin gradually rounding below and forming a con- 

 25 B H 



