438 GEOLOGY OF THE BLACK HILLS. 



form, with a very low, truncated spire, which consists of about three or 

 three and a half volutions, the outer margins of which are bordered by an 

 elevated knife-edge carina, giving a deeply channeled convolute character 

 to the spire, the center scarcely rising above the level of the outer carina- 

 tion. Body of the outer volution very slightly rounded near the margin of 

 the flattened spire and distinctly so to the base below. Aperture as long 

 as the shell, narrow above and gradually widening and rounded at the base. 

 Columella curved and apparently reflected upon the preceding volution. 

 Axis imperforate. 



Surface of the shell marked by very fine, revolving, impressed lines, 

 strongest near the top of the volution and on the lower third; also by finer 

 transverse striae of growth parallel to the margin of the aperture, and indi- 

 cating by their direction that the central portion of the outer tip is a little 

 in advance of the top and base. 



Formation and locality. — In limestone near the top of the Fort Pierre' 

 Group, on the Chyenne River, near French Creek, Black Hills, Dakota. 



PROSOPOCEPHALA. 



SOLENOCONCH^. 



DENTALIID^. 



Genus DENTALIUM Linn. 

 DENTALIUM GEACTLE. 



Plate 12, fig. 26. 



Dentalium gracile H. & M., Mem. Am. Acad. Arts and Sci., vol. v., new ser., p. 303, PI. 3, 



fig. 11. 

 Dentalium fragile M. & H., Proc. Acad. Nat. Sci. Phil., vol. 8, p. 69. 

 Dentalium gracile (H. & M.), Pal. U. S. Geol. Surv. Terr., p. 266, PI. 18, fig. 13. 



Shell slender, tubular, very gradually and regularly tapering through- 

 out, circular in a transverse section and moderately thick; very general]) 7 

 arcuate from end to end, the degree of flexure in the length of one inch, 

 measured on the concave side, being less than a sixteenth of an inch. 



