CRETACEOUS FOSSILS. 447 



ceding volution of that species; giving to tliesc a very innnature aspect 

 The finer character of the costa of this s])ecies depends in a great degree 

 upon the smaller size of the shell, as there are about the same number of 

 small costa3 between each of the stronger ones ; but there are also one or 

 two more of the stronger folds on this sj)ecies, on the deflected portion of 

 the shell, than in that one. 



The principal differences, therefore, between the two shells, and those 

 which we deem to be of specific importance, are, the greater length of tlic 

 deflected part, the laterall}'^ compressed form of the volution, and the dif- 

 ferent direction of the costoc on the straight part of the shell. Tlie two 

 forms are associated at the same localities, and even occur in the same hand 

 specimen of rock ; but we think there is no difficulty in distinguishing 

 them. The examples before us present strong variations and points of dif- 

 ference, when compared with the figures of the variety given by Mr. Meek 

 (Paleontology of the United States Geological Survey of the Territories, 

 page 421, Figs. 61, 62), especially in the straightness of the deflected por- 

 tion and in the more compressed form of the volution ; in fact these varia- 

 tions from his figures are so great, that we were in doubt of the correctness 

 of the identification, until we submitted an example to the author's inspec- 

 ion, by whom they were pronounced positively identical. We are of the 

 opinion, however, that they are specifically distinct from S. tvarreni, and 

 have so classified them. 



Formation and localitij. — In shales and limestones of the Cretaceous, on 

 the Belle Fourche, and on the east fork of Beaver Creek, Black Hills. 

 Probably of the Fort Benton Group. 



Genus HELICOCERAS D'Orb. 

 HELICOCERAS STEVENSONI.* 



Plato 14, figs. 5-8. 



Helicoceras Stevensoni Whitf., Prelim. Eept. Pal. Black Hills, 1877, p. 39. 



Shell large and robust, with an elevated, moderately tapering spire, 

 which is composed of strong, cylindrical, disconnected, dextrally-coiled 



•Named in honor of Prof. Jobu J. Stevenson, of New York City, the eminent geologist, who has 

 done so niuch good work among the Cretaceous rocks of the West. 



