CRETACEOUS FOSSILS. 385 



ventricoso, projecting" considerably above the cardinal border, slig-litly 

 influx cd 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 Pierr^ Group, on 

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

 Woman Fork, Black Hills. 



PTEEIA (OXYTOMA) NEBRASCANA. 



Plate 7, tig. 4. 



Avicula nebroMcana E. & Shumard, Trans. St. Louis Acad. Sci., voL i, p. 38. 

 Pteria nebrascana (E. & S.) Meek, Smithsonian Inst. Check List Inv. Foss., p. 0. 

 Pferia {Oxytoma) nebrascana (E. &. S.) Meek, Pal. U. S. Geol. Surv. Terr., p. 34, PI. 10, 

 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, obtuseh' 

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

 25 B H 



