440 GEOLOGY OF THE BLACK HILLS. 



ciitical examination. The principal variation consists in the more nearl}- 

 central position of the si[)hnncle and slightly more distant septa. These 

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

 ditlerent indi\iduals of a species when obtained from one locality, and we 

 are therefore strong!} inclined to doubt the proi)riety of separating it, even 

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

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

 we have figured the sij)hunclc is so nearly central in its position in the 

 fourth sc'])tuin from the outer chamber as not to be readily detected by 

 measurement, a\ hile on the opposite side of the shell, in the smaller part of 

 the same volution, its j)osition is much nearer the ventral border, showing 

 in the same individual as great a variation as between those refen-ed to the 

 two forms. 



For))iotio)i and locality. — In limestone of the Fort Pierr^ Group of the 

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



AMMONITID^. 



Genus PRIONOCYCLUS Meek. 

 PRIOXOCYCLUS WYOMINGENSIS. 



Ammonium i^Prlonocydm) serrato-carinatm Meek, GeoL Surv. Terr., 1870, p. 298 (Hayden). 



AiiimonUcs ncrratocarinatus Meek, ibid. 



Xot Ammonites .serratocariiinlu.s Stoliczka. 



Prionocychis icyomingensis 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 

 c(uivex with increased age, and the keel strongly developed, rounded, and 

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

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

 but considerabl}' more w hen 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. 



