336 GEOLOGY OF THE BLACK HILLS. 



LINGULEPIS CUNEOLUS. 



Plate 2, figs. 5, 6. 

 Lingidepis cuneolus WMtf., Prelim. Eept. Pal. Black Hills, 1877, p. 8. 



Shells small, rather below the medium size, triangularly ovate in out- 

 line, or sometimes subcuneate; ventral valve triangularly ovate, with a sharp 

 somewhat pointed beak, the width and length about as three and four, and 

 the point of greatest width near the lower third of the length of the valve; 

 cardinal slopes abrupt, scarcely convex ; basal line rounded at the sides, 

 but nearly straight in the middle ; surface of the valve strongly convex, 

 becoming almost subangular in the upper part ; dorsal valve much shorter, 

 proportionally, than the ventral, the length but little exceeding the width; 

 side and base more rounded and the beak truncate ; surface distinctly 

 convex. 



Surface of the shell apparently smooth, but usually exfoliated, in 

 which condition a few fine radiating lines are visible. 



This shell differs from L. pinnaformis Owen in size and in the more 

 distinctly cuneate form of the ventral valve. At first sight it might be 

 taken for the young of that species, but a little critical examination soon 

 reveals marked distinctions in the truncation of the front margin, and 

 especially in the form of the cardinal slopes, which are rounded and never 

 concave, as in almost all ventral valves of that species. In consequence of 

 this latter feature the beak does not appear so attenuated as in that one, 

 although proportionally quite as long. In the general surface characters 

 and form of the valves it corresponds with the genus Lingulepis, although 

 we have not been able to distinguish the muscular impressions. 



Were it not for the difference in the size of the shells, we should be 

 inclined to think this might be the species figured by Messrs. Meek and 

 Hayden (Paleontolog}^ of the Upper Missouri, p. 3, Plate 1, Fig. 1 a and b) 

 as L. dakotensis; but as their figures appear to be of the natural size, we 

 should suppose it to be very distinct, as all the specimens of our shell 

 noticed have been small, none exceeding one-fourth of an inch in length. 



Formation and locality. — In soft, friable sandstone of the Potsdam Group, 

 at Red Canon Creek, in the Black Hills, Dakota. 



