JURASSIC FOSSILS. 375 



ANATINID^l. 



Genus THRACEA Leach. 

 THEACEA 1 SUBLEVIS. 



Plate 5, fig. 34. 



Tliracea f 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 convexity cannot well be determined There can be no doubt of 

 their identity with those figured by Meek and Hay den 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. 



