180 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. loo. 



"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. vcspertinum 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 represented by two nodes, and these 

 of different forms, 1 can not believe it the same species. 



u Locality and position, — Head of Wind river valley, Wyoming; from 

 the Fort Benton group of the upper Missouri river Cretaceous series." 



It is not improbable that this species, and possibly M. vcrmilionensc 

 also, is identical with Amm. vcspcrtinus Morton (=Amm. texanus Koem.), 

 as Meek suggested, but until other collections are made from the upper 

 Missouri localities I prefer to leave them under separate names, giving 

 copies of Meek's descriptions. 



MORTONICERAS VERMILIONENSE M. & H. 



PI. xliv, Fig. 1. 



Ammonites vermilwnensxs Meek and Hayden, 1860, Proc. Acad. Nat. Sci. Pliila., p. 77. 

 Mor to nicer as vermilionense Meek, 1876, U. S. Geol. Sur. Terr., vol. ix, p. 450, PI. 7, Fig. 

 2a, o. 



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

 wider than the last turn; volutions increasing gradually in size, Avith 

 convexity about three-fourths the dorso-ventral diameter, each turn 

 less than one-fifth embraced by the succeeding 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, equal or subequal, nearly simple 

 lateral branches, the two terminal of which are diverging and moder- 

 ately 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 about 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 indentation, causing a faint tendency to a bilobate out- 

 line 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 



