surface characters and form of the valves it corresponds with the genus 

 Linyulcpis, 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 in- 

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

 Haydeu, (Pal. Upp. Missouri, p. 3, PI. 1, fig. 1 a and &,) as. 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 andloeality. In soft, friable sandstone of the Potsdam group, 

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



LTNGULEPIS PERATTENUATUS, n. si). 



Plate 2, figs. 7-9. 



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 con - 

 vex. 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 elon- 

 gated in proportion to the size of the body of the shell, but not so nar- 

 row. The two species are associated in the same blocks, and the dis- 

 similarity between examples 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 Cafion Creek, southwest Black 

 Hills, Dakota. 



