432 (lEOLOGY OF TUE BLACK UlLLS. 



Siibstanco of tlio shell thin; surface marked by fine closely-arranged revolv- 

 inf<; strijc and by transverse lines of g-rowth, which give an undulating 

 character to the revolving lines. 



Formation and locuUti/. — In limestones of the Fort Pierre Group, on the 

 Cheyenne River, near Box Elder Creek, Black Hills. 



TROCHIDJi:. 

 Genus MARGARITA Leach. 



MARGARITA NEBRACENSIS. 



Turbo nebracensis M. & H., Proc. Acad. Nat. Sci. Phil., vol. 8, p. (54. 



Margarita nebracensis M. & H., Proc. Acad, Nat. Sci. Phil., a'oI. 12, p. 185. 



Maryarita nebracensis M. & H., Pal. U. S. Geol. Siuv. Terr., p. 298, PI. 19, tigs. 8 and 9. 



Shell small, turbinate, height and width nearly equal; spire reguhirly 

 conical, composed of from four to five rounded, ventricose volutions, which 

 increase gradually in size with the increased growth of the shell, the last 

 one appearing very slightly subangular just below the middle; suture line 

 deep and well marked ; umbilicus apparently of moderate size (somewhat 

 obscured in the specimen used), and rather angular at the margin. Surface 

 marked by very fine, even, and quite regular revolving lines, and by a 

 narrow, very obscurely depressed space just above the angularity of the 

 outer whorl ; also by fine, transverse striae, which are gathered into small 

 folds just below the suture and near the margin of the umbilicus, and have 

 a strongly oblique backward direction above the periphery, and cross more 

 directly below. 



Formation and localifi/. — In limestone of the upper part of the Fort 

 Pierre Group, on the Yellowstone River, 150 miles above its mouth. 



