366 GEOLOGY OF THE BLACK HILLS. 



tumid beaks situated a little nearest to the anterior end, and projecting 

 above the general line of the hinge; hinge line, posterior to the beaks, 

 about half as long as the entire length of the shell, straight, but gradually 

 declining to the extremity; posterior end of the shell very obliquely trun- 

 cate, being much the longest below the middle at the postero-basal angle; 

 anterior end shorter, slightly contracted in front of the beaks, but rounded 

 at the extremity and broader than the opposite end ; basal margin straight 

 or very slightly convex in the middle and strongly rounded upward toward 

 each extremity. Body of the shell often distinctly angular along the pos- 

 terior umbonal ridge and the postero-cardinal slope quite abrupt; some 

 individuals, however, are obtusely rounded on the umbonal ridge. Surface 

 of the shell marked by rather strong concentric ridges of growth, which 

 are crossed on the rounded anterior end and middle of the valve by 

 extremely fine radiating striae, only detected near the basal portion of the 

 larger individuals by the aid of a moderate magnifying power. 



There may be some little doubt as to the true generic relations of this 

 shell, but it possesses so many of the features of Trapezium that we have 

 not hesitated to refer it to that genus; the almost subcentral and somewhat 

 prominent beaks forms the strongest point of difference. It differs in this 

 respect from the preceding species, as also in the general form of the valves 

 and in the surface markings The shells of the two species have nearly the 

 same dimensions, and in their mutilated and imperfect condition, as seen 

 imbedded in the rock, may readily be confounded, but the more prominent 

 beaks and longer anterior end of this one will at once distinguish them. 



Formation and locality. — In a hard white sandstone of Jurassic age, 

 associated with Pseudomonotis curta, just above the Triassic red beds at Red 

 Canon Creek, Black Hills, Dakota. 



