370 GEOLOGY OF THE BLACK HILLS. 



Genus NEtERA Gray. 

 NE^EA LONGIEOSTEA. 

 Plate 5, fig. 35. 

 Necera longirostra Wlritf., Prelim. Eept. Pal. Black Hills, 1877, p. 24. 



Shell very small, scarcely exceeding one-fourth of an inch in length ; 

 transversely elongate- ovate or clavate, widest anterior to the middle and 

 narrowed behind, a little more than twice as long as high. Valves very 

 convex, becoming inflated near the middle and on the umbones. Beaks 

 proportionally large, erect, strongly projecting above the hinge line and 

 situated in front of the middle of the length ; anterior end large and 

 broadly rounded, somewhat excavated beneath the beaks ; posterior end 

 narrowed, elongate, and very slightly recurved, and obliquely truncate 

 above the umbonal angle ; basal line strongly convex, more distinctly so 

 just in front of the middle opposite the beaks, and broadly constricted 

 behind ; posterior umbonal ridge very faintly angular, and the cardinal 

 slope rather abrupt. 



Surface of the shell marked by a few concentric lines of growth, and 

 crossed on the anterior and middle parts of the shell by a few proportion- 

 ally strong elevated radiating ridges, to the number of eight or nine, with 

 wider flattened interspaces. 



The species is small, and being preserved in a friable sandstone the 

 surface characters are not clearly defined, nor can any of the hinge or 

 other internal features be seen. 



Formation and locality. — In white sandstones of Jurassic age in Red- 

 water Valley, Black Hills, associated with Pseudomonotis curta, Tancredia 

 warrenana, and other Jurassic fossils. 



GASTROCLLENIim 



Genus SAXICAVA Bellerue. 

 SAXIOAVA JUEASSIOA. 



Plate 5, figs. 25-30. 



Saxicava jurassica Whitf., Prelim. Eept. Pal. Black Hills, 1877, p. 25. 



Shell small, elongate-subcylindrical, with subparallel dorsal and basal 

 margins ; beaks approximate, distinct, situated near the anterior end, and 



