364 GEOLOGY OF THE BLACK HILLS. 



anterior third of the length, and gradually decreasing in convexity poste- 

 riorly. Beaks scarcely elevated above the cardinal line, small, pointed, 

 strongly directed forward, and situated rather within the anterior fourth of 

 the entire length of the shell. Anterior end short and rounded, the longest 

 point opposite or a little below the middle of the height; basal line regularly 

 and gently arcuate; posterior end more sharply rounded than the anterior, 

 longest below the middle, the cardinal line sloping with a gentle convex 

 curvature from the beaks to the longest point. No appearance of escutcheon 

 or hi mile exists on the examples under consideration. Ligament very 

 small and external. 



We have no hesitation in referring the specimens under consideration 

 to the above-named species, but they certainly are not congeneric with the 

 recent forms of Astarte. They seem to us to be much more nearly related 

 to some of those referred to the genus Cykena, especially such as C. papua 

 Prime; but as the internal features of the hinge have not been observed, it 

 is impossible to fully determine their generic relations; and we have there- 

 fore left the generic reference as originally given by the authors of the 

 species. 



Formation and locality. — In shaly limestones of Jurassic age, at a 

 horizon 350 feet above the Red Beds, at Redwater Valley, Black Hills. 



Genus TRAPEZIUM Humph. (= CYPRICABDIA Lam.) 



TBAPEZIUM BELLEFOTJECHENSIS. 



Plate 5, figs. 1-4. 

 Trapezium bellefourchensis WMtf., Prelim. Bept. Pal. Black Hills, 1877, p. 18. 



Shell small, transversely elongate quadrangular, the length being from 

 two and a half to three times greater than the height: dorsal and basal 

 margins subparallel, the former very slightly arched and the latter scarcely 

 concave, except in the older individuals, the smaller and medium-sized 

 individuals being usually a little curved upward toward the posterior end; 

 beaks small, nearly terminal, rounded on the umbones and very slightly 

 incurved; posterior extremity truncate in the older specimens, rather 

 squarely so below and rounding forward to the extremity of the hinge line 



