426 GEOLOGY OF THE BLACK HILLS. 



ijij^ lines of the surface ; callosity thin and smooth, without tubercles. No 

 vertical folds exist. 



The .species is so entirely distinct from any of the described forms of 

 this group of shells from our American rocks, that an elaborate comparison 

 .seems unnecessary. 



Formation and locality. — In limestones of the Fox Hills Group, on the 

 Rio de la Plata. Collection of Dr. J. S. Newberry. 



APORRHAIS MEEKI. 



Plate 12, lig. 5. 



Aporrhais meeli Wliitf., Prelim. Kept. Pal. Black Hills, 1877, p. 37. 



Shell of moderate size, with a low, very obtusely-pointed spire, which 

 is compo.sed of only three ventricose or rounded volutions, exclusive of the 

 apertural expansion, and which rapidly increase in size; apical angle nearly 

 80° Volutions marked b}' numerous sharply-elevated, revolving lines, 

 with concave interspaces both above and below the middle. Outer lip 

 expanded and extended into two diverging carinated digitations of an unde- 

 termined length, which are deeply grooved on the inner face, the upper one 

 being directed slightly backward toward the spire, and the other one 

 slightly downward and more strongly forward in the direction of the coil of 

 the shell; anterior beak, judging from the part remaining, moderately long 

 and stout ; posterior canal extending along the spire to the summit, where 

 it appears to have become free and deflected ; callous slight (?), coating the 

 upper volutions over a part of their extent. 



This species differs from any previously-described form of this group in 

 the short obtuse spire, combined with the two digitations of the outer lip, 

 and the posterior canal, which extends to the summit of the spire. 



Formation and locality. — In limestone of the Fort Pierre Group, at the 

 top of the gray shales of this formation, on the Cheyenne River near Rapid 

 Creek, Black Hills. 



