370 (iKOLOCJY OF TH1-: BLACK [IILLS. 



TANCREDTA CORBULIFORMIS. 



Plato 6, figs. 5-8. 

 Tancredia corbuli/ormis Whitf., Prelim. Rept. Pal. Black Hills, 1877, p. 21. 



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

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

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

 full, slightly enrolled, almost central and strongly inclined toward the ante- 

 rior 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 curved, except at the 

 posterior part, where it becomes a little more abrupt. Body of the valve 

 strongly convex, almost inflated on the umbo. 



Surface of the shell apparently smooth, but under a magnifier showing 

 faint, scarcely defined, lines of growth. 



The species differs from T. inornata M. & H., in its smaller size ; more 

 inflated valves; proportionally larger and more prominent beaks, and 

 greater inequality in size of the anterior and posterior parts of the shell. 



Formation and locality. — In sandy and somewhat shaly limestone of 

 Jurassic age, 350 feet above the red beds of the Friassic, east of the Belle 

 Fourche River, near Bear Lodge Butte, Black Hills. 



TANCREDIA BULBOSA. 



Plate 6, figs. 1-3. 



Tancredia bnlbosa Whitf., PreUrn. Rept. Pal. Black Hills, 1877, p. 22. 



Shell very small, the larger individuals not measuring more than three- 

 eighths of an inch in length by a height of one-fourth of an inch; very 

 broadly ovate in outline, being inflated and globuliform in the middle and 

 posteriorly, and attenuate, narrowed and compressed toward the anterior 

 extremity. Beaks strong, inflated, curving anteriorly and situated a little 

 to the anterior of the middle of the shell; anterior end acutely rounded, 

 the sides of the shell abruptly compressed b}' a broad, undefined, constrict- 

 ing sulcus, or depressPon, which passes obliquely across the valve from in 

 front of the beaks to the basal border, in which it forms a distinct emargi- 



