368 GEOLOGY OF THE BLACK HILLS. 



concentric lines. No appearance of escutcheon or lunule exists, nor of an 

 external ligament. 



Surface of the shell marked by numerous irregular concentric undula- 

 tions, which are usually broad and undefined, but on some individuals have 

 a slightly sharper character. The general surface beyond these undefined 

 undulations appears to have been smooth, and we should suppose from its 

 character that it had been semi-polished Substance of the shell moder- 

 ately thick. The features of the hinge and interior have not been fully 

 ascertained. There appears, however, on the left valve a strong tooth-like 

 process of considerable height, and in front of it a deep triangular pit, 

 probably ligamental. 



This species differs from Myacites (Pleuromya) subcompressa, Meek (Sixth 

 Annual Report United States Geological Survey of the Territories, 1872, p. 

 472), in being proportionally longer; more distinctly ovate or less triangu- 

 lar, the anterior end being of greater length in proportion to the rest of 

 the valve; the beaks are larger and project farther above the cardinal 

 line; the body of the shell is less flattened and the surface less strongly 

 marked. 



Formation and locality. — In shaly limestones of Jurassic age, on top of 

 the highest hills two miles south of the Belle Fourche River, near Bear 

 Lodge Butte, Black Hills. 



TANCREDinm 



Genus TANCREDIA Lycett {-HETTANGIA Terquem.) 



TANCEEDIA % INOENATA. 

 Plate 6, figs. 9-13. 



Astarte inornata M. & H., Proc. Acad. Nat. Sci. Phil., May, 1860, p. 183. 

 Astarte inornata M. & H., Pal. Up. Missouri, p. 94, PL iii, fig. 12, a. b. 



Shell transversely subelliptical, or transversely broad-triangular ; ex- 

 tremities narrowly rounded and the basal border forming a broad, nearly 

 regular semi-elliptical curve. Valves convex, most ventricose along the 

 posterior umbonal portions. Beaks large, prominent, nearly subcentral and 

 incurved, usually a little nearer the anterior end ; cardinal line strongly 



