406 GEOLOGY OF THE BLACK HILLS. 



there are small, irregular, intermediate transverse teeth in the center. These 

 we have not seen, our specimens not showing this part of the hinge plate.) 

 Hinge area of moderate height, marked by distinct, oblique, ligamental 

 grooves, widely diverging from the center of the area; anterior end of the 

 shell broadly and regularly rounded, and imperceptibly passing into the 

 basal line; posterior end obliquely truncate, and directed backward to the 

 posterior extremity, below the middle of the height of the valve; basal 

 border more broadly rounded. 



Surface of the valves marked by fine concentric lines of growth and 

 by very fine indistinct radiating lines, which form pits at the concentric 

 furrows, seen only by the aid of a lens. Anterior muscular imprint large 

 and subcircular, situated close within the margin and near the extremity of 

 the hinge line; posterior impression obliquely elongated, gradually increas- 

 ing- in width from above, and distinctlv bordered on the inside bv a thin 

 elevated ridge, as in Cucullcea. 



Formation and locality. — In beds of the Fox Hill Group of the Cre- 

 taceous, on the Cheyenne River, near Old Woman Fork, Black Hills. 



Genus NUCULA Lam. 



STTTCULA PLANIMAEGINATA. 

 Plate 11, figs. 5, 6. 



Nucida planimarginata M. & H., Proc. Acad. Nat. Sci. Phil., vol. 8, p. 85, ib., Nov. 



1856, p. 21. 

 Nucula planimarginata (M. & H.) Meek, Smithsonian Check List Invert. Foss., p. 8. 

 Nucula planimarginata (M.& H.) Meek, Pal. U. S. Geol. Surv. Terr., p. 101, PI. 15, fig. 8, 



and PI. 28, fig. 16. 



Shell rather above the medium size, transversely-ovate, about two- 

 thirds as high as long, with convex valves and somewhat prominent beaks, 

 which are slightly incurved, and placed within the anterior third of the 

 length of the valve; anterior end short, obliquely truncate, the longest 

 point rather below the middle of the height, and rather sharply rounded ; 

 posterior end long and more broadly rounded than the anterior, basal 

 margin strongly curved, having a semi-oval outline; cardinal line behind 

 the beaks gently arcuate. 



Surface of the shell not clearly determined, but, as far as can be ascer- 



