414 GEOLOGY OF THE BLACK HILLS. 



VENIELLIID^]. 



Genus VENIELLA Stoliczka. 

 VEMELLA HUMILIS. 



Plate 10, figs. 7-13. 



Cyprina humilis M. & H, Proc. Acad. Nat, Sci. Phil. May, 1860, p. 179. 

 Venilia humilis (M. & H.) Meek, Smithsonian Check List, Invert. Foss., p. 13. 

 Yeniella humilis (M. & H.) Meek, Pal. IT. S. Geol. Surv. Terr., p. 155, PI. 30, fig. 5. 



Shell of moderate size ; transversely oblong 1 or trapezoidal, or trans- 

 versely-oval ; the length considerably exceeding the height and often, in 

 casts, the condition in which they are usually obtained, much less than 

 two-thirds as high as long ; the thickness through the united valves but 

 little less than the height, which is greatest at about the anterior third of 

 the length. Beaks large, tumid, inflated on the umbones, slightly angular, 

 strongly incurved and approximate, directed forward, and situated nearly 

 vertically above the anterior end of the shell ; hinge line arcuate, much 

 shorter than the length of the shell and gradually declining toward the 

 posterior extremity, which is obliquely truncate and longest below ; basal 

 line somewhat regularly curved; anterior end rounded. Umbonal ridge 

 angular or more or less rounded angular, and strongly arcuate in its direction 

 from the beaks to the postero-basal angle of the valve. Ligament external 

 and of moderate size. Surface covered by strongly marked lines of growth. 

 Internally, the muscular impressions are distinctly marked, the anterior 

 very deep. Cardinal teeth strong, lateral tooth having the form of an 

 elevated angular ridge fully one-third as long as the valve. 



The species is a strongly marked and rather abundant one, but is quite 

 variable in its proportions in different individuals ; the length and height, 

 as seen on casts, varying from once and a half to nearly twice the height. 

 The substance of the shell is thick and massive, especially along the car- 

 dinal border, and when entirely removed the casts appear much more elon- 

 gated in proportion than when the shell is preserved. 



Formation and locality. — In ferruginous sandstone of Cretaceous No. 

 5, on the Cheyenne River, near Old Woman Fork, Wyoming. 



