22 



little more abrupt. Body of the valve strongly convex, almost inflated 

 on the umbo. 



Surface of 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 shaiy limestones 

 of Jurassic age, 350 feet above the red beds of the Triassic, east of 

 Belle Fourche, Black Hills, Dakota. 



Tanckedia bulbosa, n. sp. 



Plate 6, figs. 1-3. 



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. Beak strong, inflated, curving anteriorly, and sit- 

 uated a little to the anterior of the middle of the shell; anterior end 

 acutely rounded, the sides of the shell abruptly compressed by a broad, 

 undefined, constricting sulcus, or depression, which passes obliquely 

 across the valve from in front of the beaks to the basal border, in 

 which it forms a distinct emargination ; posterior end broadly rounded, 

 and the posterior cardinal line strongly convex ; anterocardinal line 

 constricted in front of the beaks, and concave between them and the 

 anterior extremity;' basal line very convex for a little more than two- 

 thirds of the length from the posterior end, where it becomes emargi- 

 nate at the base of the oblique sulcus or depression, which crosses the 

 anterior part of the valve. Surface semi-polished, and under a magni- 

 fier showing fine lines of growth. 



The species resemble T. corhuliformis herein described, but is a smaller 

 , shell, seldom occurring of more than half the usual length of that one, 

 and is much more extreme in the unequal size of the anterior and pos- 

 terior parts of the valve, besides being much more globose in form, and 

 having the anterior oblique sulcus of the valve much more strongly 

 marked. 



Formation and locality. — In rock of Jurassic age associated with T. 

 corhuliformis and T. inornata at the same locality. 



TANCREDIA POSTICA, 11. sp. 



Plate 6, fig. 14. 



Shell small, transversly ovate, the height and length being about as 

 three to five. Valves very ventricose, becoming tumid near the beaks 

 and on the postero-cardinal part. Beaks large, and but little elevated 



