JURASSIC FOSSILS. 371 



nation; posterior end broadly rounded and the postero- cardinal line strongly 

 convex; antero- cardinal 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 

 emarginate 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 resembles T. corbuliformis 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 posterior 

 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. cor- 

 buliformis and T. inornata, at the same locality. 



TANOREDIA POSTICA. 



Plate 6, fig. 14. 

 Tancredia postica Wkitf., Prelim. Rept. Pal. Black Hills, 1877, p. 22. 



Shell small, transversely 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 above the cardi- 

 nal line, slightly enrolled and directed forward, situated at about one-third 

 of the length from the anterior extremity; antero-cardinal line abruptly 

 declining in front of the beaks to the middle of the height of the valve; 

 anterior end narrow and sharply rounded; posterior end very broadly 

 rounded and the postero-cardinal and basal margins subparallel behind the 

 beaks, the extreme height of the shell continuing to the posterior end; 

 antero-basal line strongly curving upward to the anterior end of the shell. 

 Surface of the valves apparently smooth. 



The shell differs from any of the preceding species in the greater 

 length and proportionate size of the posterior portions of the shell, in the 

 situation of the beaks, and in their strong anterior inclination, the others 

 being much more erect than those of this species. It will be readily recog- 



