412 GEOLOGY OF TOE BLACK HILLS. 



is constricted, forming- a slight fold within the margin. Surface of the shell 

 marked by fine concentric striai ; ligament of moderate size, deeply seated 

 within the borders of the valves, which are not inflected for its reception, 

 but are distinctly so to form the short, deep lunule on the anterior side of 

 the beaks. 



The species is associated with the two preceding ones, but differs very 

 materially in the details of structure, although closely resembling- them in 

 general form. The sulcus of the shell on the anterior border would place 

 it in a different section of the genus from either of those; and the orbicular 

 outline, narrow cardinal slope, deeply seated lunule, and more prominent 

 beaks will also serve to readily distinguish them. 



Formation and locality — At the top of the gray shales of the Fort 

 Pien'd Group, on the Cheyenne River, near Rapid Creek, Black Hills, 

 Dakota. 



CRASSATELLIDiE. 

 Genus CRASSATELLA Lam. 



CRASSATELLA SUBQUADRATA. 

 Plate 11, fig. 12. 



Crassatella stibqtiadrata Wbitf., Prelim. Reprt. Pal. Black Hills, 1877, p. 34. 



Shell small, subquadrangular in outline, with thin compressed valves 

 and not prominent beaks, which are situated within the anterior third of the 

 length; length of the shell exceeding the height; basal margin broadly 

 rounded; anterior end short and narrowly rounded; posterior end broad 

 and rounded truncate; posterior hinge line straight and subparallel to the 

 basal margin Surface of shell unknown. Muscular impression small; the 

 posterior one situated close beneath the extremity of the hinge margin, and 

 the anterior scar submarginal. Pallial line not distinctly marked; margin 

 of the valve crenulate. 



The only specimen of this genus recognized among the collections 

 from the Black Hills, is a small internal cast, answering to the above 

 description, ;ind apparently a true Crassatella. Although the hinge plate 



