21 



narrowed, tlie postero-cardinal naargia rapidly decliniug from the ex- 

 tremity of the hinge-line to the longest point of the valve, which is 

 narrowly rounded and at about the middle of the height ; basal margin 

 very slightly constricted, or emarginate, just within the anterior third 

 of the length, by a faint sulcus, which passes from the surface of the 

 umbo to the base of the shell ; posterior to this it is strongly convex, 

 and as it approaches the posterior third of the length is more strongly 

 rounded upward to the extremity. Body of the valve convex, most 

 ventricose on the umbones and along the middle of the valve; anterior 

 umbonal ridge obtusely angular; anterior slope abrupt and sometimes 

 flattened obliquely ; posterior umbonal ridge scarcely defiaed or broadly 

 rounded. Immediately behind the anterior umbonal ridge a broad 

 undefined sulcus passes across the valve, slightly modifying the direction 

 of the concentric lines. No appearance of escutcheon or lunule exists, 

 nor of an external ligament. 



Surface of the shell marked by numerous irregular concentric undu- 

 lations, which are usually broad and undefined, but on some individuals 

 have a slightly sharper character. The general surface beyond these 

 undefined undulations appears to have been smooth, and we should 

 suppose from its character that it had been semipolished. Substance 

 of the shell moderately thick. The features of the hinge and interior 

 have not been fully ascertained ; there appears, however, on a left valve 

 a strong tooth-like process of considerable height, and in front of it a 

 deep triangular pit, probably ligamental. 



This species differs from Myaoites (Pleuromya) subcornpressa, Meek 

 (Sixth An. Eept. U. S. Geol. Surv. Territories, 1872, p. 472), in being pro- 

 portionally longer; more distinctly ovate or less triangular. The ante- 

 rior end being of greater length in proportion to the rest of the valve, 

 the beaks are larger and project farther above the cardinal line; 

 the body of the shell is less flattened and the surface less strongly 

 marked. 



Formation and locality. — In shaly limestone of Jurassic age, on top of 

 the highest hills two miles south of Belle Pourche, Black Hills, Dakota. 



Tancredia corbulifomis, n. sjp, 



Plate 6, figs. 5-8. 



Shell small, seldom measuring more than half an inch in length, very 

 broadly ovate in outline and largest on the posterior side of the beaks, 

 the length being a little more than once and a half the height ; the beaks 

 large and full, slightly enrolled,- almost central and strongly inclined to- 

 ward the anterior part of the shell. Anterior end narrowed, strongly 

 constricted in front of the beaks, giving a somewhat concave antero- 

 cardinal line, the. extremity acutely rounded; posterior end broadly 

 rounded, and the postero cardinal line convex to near the beaks ; basal 

 line regularly carved except at the posterior part where it becomes a 



