484 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



tion. The surface is ornamented by distinct, rounded, oblique, flexuous, 

 annular costse, which sometimes bifurcate on the lower outer side, where 

 they support two parallel rows of obtuse nodes. 



This fragment is referred provisionally to this genus, upon the suppo- 

 sition that it belongs to the deflected portion of the shell, which view is 

 sustained by its large size, very broad curve, and non-septate condition. It 

 may be the free part of the species we have described under the name H. 

 umbilicatum, or some of the others, but we have no direct evidence of this. 

 At some future time, when more complete specimens have been found, these 

 doubtful points will be more satisfactorily settled. Its spiral curve shows 

 that it cannot belong to Ancyloceras or Hamites ; and the fact that it belongs 

 to a shell with the volutions partly or entirely free shows that it cannot be 

 properly included in the genus Turrilites as restricted by the separation of 

 •the genus Heteroceras. 



This shell seems to be related to AmmonitesX Cooperi, Gabb (Cal. Geo- 

 logical Rep., I, 69, pi. xiv, fig. 23), which I think must be at least congeneric. 



Locality and position. — Mouth of Big Cheyenne River, Dakota; from 



the upper part of the Fort Pierre group of the Upper Missouri Cretaceous 



series. 



Heteroceras? a u g u I a I u m , M. & H. 



Plato 21, fig. 3, a, 6, e. 



Helicoccras angulatum, Meek and Hayden (1860), Proceed. Acad. Nat. Sci. Philad., XII, 176. 

 Heteroceras 1 angulatum, Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 25. 



Of this shell we have a single non-septate fragment, 2.78 inches in 

 length, with a diameter of 1.50 inches at the larger end, and 1.37 inches at 

 the smaller extremity. It is rounded, or subcylindrical, and makes a broad 

 (dextraH) spiral curve, in such a manner that if continued around, the volu- 

 tions would be disconnected, and encircle an umbilical cavity apparently 

 more than three times their own breadth. The surface is ornamented by 

 distinct angular costse, which pass around the whorls obliquely, and support 

 two rows of nodes on the lower outer side, where they sometimes bifurcate. 



I have not yet seen the septa of this species, but its large size and very 

 broad curve will distinguish it from any other known species of this type in 

 these rocks, excepting the last, from which it differs in having angular 

 instead of rounded costa?, and in being coiled in the opposite direction. Its 

 angular rostse will equally distinguish it from any of the others, even should 



