472 University of Kansas Geological Survey. 



This fossil occurs in the Septaria horizon of the Fort Benton 

 formation, but so far only fragments or impressions have been 

 found. These were collected from the vicinity of William's 

 Butte, in Mitchell county. 



Mortoniceras vermilioneiise M. & H. Plate civ, fig. 1. 



Ammonites vermilionensis M. & H., 1860, Proc. Acad. Nat. Sci. Phil., p. 77. 

 Mortoniceras vermilionen.se Meek, 1876, U. S. Geol. Surv. Terr., vol. ix, p. 

 450, pi. 7, ff. 2a, b; Stanton, 1892, Bull. U. S. Geol. Surv. No. 106, p. 180, 



pi. XLIV, f. 1. 



"Shell compressed-discoid, with its shallow umbilicus about 

 one-fifth wider than the last turn ; volutions increasing gradu- 

 ally in size, with convexity about three-fourths the dorso-ventral 

 diameter, each turn less than one-fifth embraced by the suc- 

 ceeding outer one ; costse simple and closely arranged in the 

 very young shell, but gradually becoming larger, more distant, 

 and a little thickened at their inner and outer extremities, 

 which latter are slightly curved forward, in examples an inch or 

 so in diameter ; peripheral keel moderately prominent, with the 

 depression on each side shallow. Septa not crowded ; siphonal 

 lobe oblong, about one -fourth longer than wide, with two short, 

 narrow or subequal or equal, nearly simple, lateral branches, the 

 two terminal of which are diverging and moderately distant ; 

 .first lateral sinus as long and nearly twice as wide as the 

 siphonal lobe, and deeply divided into two nearly or quite equal 

 parts, with merely sinuous and obtuse digitate margins ; first 

 lateral lobe slightly longer than the siphonal, and of the same 

 breadth, with some five or six spreading unequal digitations at 

 the posterior end, the middle two of which sometimes become 

 more prominent, so as to give a slightly bifid appearance to the 

 extremity ; second lateral sinus short, or scarcely more than 

 half as long on the inner side as the first, subquadrate in form 

 with shallow marginal sinuosities ; the mesial very shallow in- 

 dentations, causing a faint tendency to bilobate outline at the 

 anterior extremity ; second lobe very small, or even less than 

 the auxiliary lobe of the first lateral sinus, about .twice as long 

 as wide, narrower, and truncated at its posterior end, with a 

 very few shallow sinuosities along its lateral margins ; third 



