440 GEOLOGY OF THE BLACK HILLS. 



critical examination. The principal variation consists in the more nearly 

 central position of the siplmncle and slightly more distant septa. These 

 features, however, are not more marked than may often be noticed among 

 different individuals of a species when obtained from one locality, and we 

 are therefore strongly inclined to doubt the propriety of separating it, even 

 as a variety, from those referred by its author to N. dehayi. We are the 

 more strongly confirmed in this opinion from the fact that in the specimen 

 we have figured the siphuncle is so nearly central in its position in the 

 fourth septum from the outer chamber as not to be readily detected by 

 measurement, while on the opposite side of the shell, in the smaller part of 

 the same volution, its position is much nearer the ventral border, showing 

 in the same individual as great a variation as between those referred to the 

 two forms. 



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

 Cretaceous, on the Cheyenne River, near Rapid Creek, Black Hills, Dakota. 



AMMONITID^. 



Genus PRIONOCYCLUS Meek. 

 PRIONOCYCLTTS WYOMINGENSIS. 



Plate 14, tigs. 1-3. 



Ammonites (Prionocyvlus) serrato-carinatus Meek, Geol. Surv. Terr., 1870, p. 298 (Hayden), 



Ammonites serrato-carinatus Meek, ibid. 



Not Ammonites serrato-carinatus Stoliczka. 



Prionocyclus ivyomingensis Meek, Pal. U. S. Geol. Surv. Terr., p. 452, note. 



Shell attaining a medium or large size, subdiscoid and laterally com- 

 pressed. Volutions flattened on the sides in the younger stages and 

 strongly keeled on the dorsum, becoming proportionally wider and more 

 convex with increased age, and the keel strongly developed, rounded, and 

 serrated. The lateral diameter of the volution of a specimen which meas- 

 ures about 1 1 inches across is about two-thirds the dorso- ventral diameter, 

 but considerably more when they attain a diameter of three inches. Umbili- 

 cus large and open, exposing four-fifths to five-sixths of the diameter of the 

 inner volutions, the margin of the volutions being abruptly rounded. 



