JURASSIC FOSSILS. 375 



ANATINIDJ]. 



Genus THRACEA Leach. 



THRAOEA ? SUBLEVIS. 



Plate 5, fig. :m. 



Thracea ? sublevis M. & H., Proc. Acad. Nat. Sci. Phil., May, 1860, p. 182; ibid., Oct. 

 1860, p. 418. 



Shell rather below the medium size, transversely elongate with mod- 

 erately convex valve, which appear to have been somewhat gaping at the 

 posterior end. Beak large and wide, projecting somewhat above the hinge 

 line, and situated a little in advance of the middle of the length ; anterior 

 end more broadly rounded than the opposite; basal line nearly straight or 

 very slightly sinuate near the middle, and rounded upward at each extrem- 

 ity ; posterior extremity more narrowly rounded than the anterior, and 

 slightly oblique on the truncation ; cardinal line sloping very moderately 

 from the beaks posteriorly, and rounding more abruptly in front ; surface 

 of the valves very slightly impressed across the middle from the beaks to 

 the base by a very broad, undefined depression ; posterior umbonal ridge 

 faintly marked, and the postero-cardinal slope rounded ; anterior slope 

 more abrupt 



Surface marked only by a few^ irregular concentric undulations of 

 growth parallel to the margin of the valve. 



The specimens examined are single valves and imperfect, so that the 

 relative convexit}^ cannot well be determined There can be no doubt of 

 their identity with those figured by Meek and Ilayden as above cited ; the 

 specimens are all of smaller size, however, but agree in all other respects. 

 They are preserved as casts in a white friable sandstone, and are associated 

 with Pseudomonotis curta, Tancredia warrenana, and olher Jurassic fossils. 



Formation and locality. — In rocks of Jurassic age, at Redwater Valley, 

 Black Hills. 



