PKIMOKDIAL FOSSILS. 337 



LINGULEPIS PERATTENUATUS. 



Plate 2, iiga. 7-9. 



Lingulepis pcrattenucutxis Whitf., Prelim. Eept. Pal. Black riilLs, 1877, p. 0. 



Shell spatulate or elongate-ovate, becoming- acutely pointed toward 

 the beak; sides below the middle of the length of the ventral valve 

 rounded; front margin very regularly curved; cardinal slopes straight, or 

 sometimes slightly convex; greatest width a little more than half the 

 length of the ventral valve and sometimes nearly two-thirds as great. 

 Surface of the ventral valve depressed convex, very slightly subangular 

 , along the middle above the lower third of the length, and a little more 

 distinctly so in the upper part. Apex scarcely truncate. Dorsal valve 

 considerably shorter than the ventral, broadly truncated at the upper end, 

 the line of truncation being distinctly arched. Surface evenly convex. 

 Structure of the surface of the shell polished, with fine, somewhat lamellose, 

 lines of growth. 



The shell, in many points, closely resembles L. pinnaformis Owen, but 

 is a proportionally longer shell, and not exceeding one-half the size of the 

 adult specimens of that species. When compared in shape, it is more 

 elongate in front, the lower end of the shell forming a section of an ellipse 

 rather than of a circle, as in that one ; the cardinal slopes are convex and 

 the valves proportionally longer, the apex being more elongated in propor- 

 tion to the size of the body of the shell, but not so narrow. The two spe- 

 cies are associated in the same blocks, and the dissimilarity between exam- 

 ples of the same size is so perceptible, that there will not be the slightest 

 difficulty in recognizing them as distinct. 



Formation and locality.— In greenish-purple sandstone of the Potsdam 

 formation, at the headwaters of Red Canon Creek, Black Hills, Dakota. 



LINGULEPIS DAKOTENSIS. 



Plate 2, figs. 10, 11. 



Idnguelepis pinnaformis (Oweu) I\r, & H., Pal. Upp. Missouri, p. 2, PI. 1, fig. 1, 

 Lingtielepis dalMtensis M. & H., Pal. Missouri, p. 3. 



" Shell subovate or ovate-subtrigonal, rounded in front and angular at 

 the beaks, moderately convex ; sides converging to the beaks with straight 



22 B H 



