450 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



inner turn covered by the succeeding volution, the inner margin of which is 

 indented by the rounded lateral nodes of that next within. 



Septa moderately approximate; sipbonal lobe oblong, about once and a 

 half as long as wide, with small, short, nearly parallel, serrated, terminal 

 branches, and three or four very short, digitate, and simple branchlets and 

 points on each side ; first lateral sinus wider than the siphonal lobe (which 

 it equals in length), unequally bipartite at the anterior end, both divisions 

 being digitate, and the larger one on the siphonal side deeply bifid ; first 

 lateral lobe somewhat longer, but narrower than the siphonal, and having its 

 terminal division deeply bitid, and its lateral margins bearing a few very 

 nearly simple branchlets ; second lateral sinus scarcely more than half as 

 wide as the first, and much shorter on the umbilical side, unequally bifid or 

 trifid at the end, with more or less sinuous margins ; second lateral lobe only 

 about half as long and wide as the first, and trilobate, with the small middle 

 division emarginate at the end ; third lateral sinus a little shorter and 

 narrower than the second, with a bipartite end and serrated margins ; third 

 lateral lobe nearly as long as the second, but narrower, and irregularly 

 tridentate at the end ; antisiphonal lobe about as long as the first lateral, but 

 narrower, with a few short, nearly simple, lateral divisions, and a tridentate 

 posterior extremity. 



Having only a fragment of this species, no measurements of the size 

 and proportions of the shell can be given. As this fragment shows the form 

 and ornamentation, however, of the volutions, and very clearly all of the 

 details of the septa, there will probably be no difficulty in identifying it. At 

 one time, I was rather inclined to regard it as a variety of M. vespertinum, 

 Morton ; but as the costae in that species seem to be very constant, in the 

 possession of four nodes each, while on the form here described each rib is 

 merely represented by two nodes, and these of different forms, I cannot 

 believe it the same species. 



Locality and position — Head of Wind River Valley, Wyoming Terri- 

 tory ; from the Fort Benton group of the Upper Missouri River Cretaceous 



series. 



Hlortoniccras! Vermilioiicnse, M. & H. 



Plato 7, fig. 2, a, b. 

 Ammonites Vermilioncnsis, Meek and Hayden (18G0), Proceed. Acad. Nat. Sci. Philad., XII, 177. 



Shell compressed-discoid, with its shallow umbilicus about one-fifth 



