372 GEOLOGY OF THE BLACK HILLS. 



nized by the inflated valve, subparallel cardinal and basal margins, and short 



anterior end. 



Formation and locality. — In shaly limestones of the Jurassic formation, 



east of the Belle Fourche River, Black Hills, associated with the preceding 



species. 



TANCEEDIA WAEEENANA. 



Plate 6, fig. 4. 



Tancredia warrenana M. & H., Proc. Acad. Nat. Sci. Phil., May, 1860, 183; ibid. : Oct. 



1860, p. 418. 

 Tancredia warrenana M. & H., Pal. Up. Missouri, p. 96, PI. 3, fig. 7. 



Shell small, subtriangular in outline, or triangularly ovate, moderately 

 convex, and nearly two-thirds as high as long. Beaks proportionally large, 

 projecting above the cardinal line, erect, pointed, and subangular; situated 

 rather more than one-third of the entire length from the posterior end of 

 the shell. Hinge line contracted in front of the beaks, and somewhat regu- 

 larly sloping anteriorly. Anterior end of the shell much the longest, 

 narrowed and compressed, rounded at the extremity, and, judging from the 

 form of the shell as seen in separated valves, has been distinctly gaping; 

 posterior end short, obliquely truncate and abrupt; basal line gently and 

 regularly arcuate throughout; posterior umbonal ridge sharply angular, 

 the posterior cardinal slope narrow and abruptly declining. Surface un- 

 known. 



The specimens of this species present in the collection are casts of the 

 interior on layers of fine white sandstone, and do not preserve the surface 

 features or any of the internal features of the shell. 



Messrs. Meek and Hayden refer this species provisionally to the genus 

 Trancredia, remarking that its form leaves little doubt of its relations to that 

 genus. Our specimens are not in a condition to give any additional light 

 on its generic relations, and we are therefore obliged to leave it as originally 

 placed by its authors. 



Formation and locality — In soft white sandstone of Jurassic age, asso- 

 ciated with Pseudomonotis curta Hall and other Jurassic forms, at a horizon 

 of 135 feet above the red beds of the Triassic, at Eedwater Valley, south- 

 east of Warren Peaks, Black Hills. 



