INVERTEBRATE PALEONTOLOGY. (509 



one of which is longer than the others, a little oblique, and not exactly central. 

 The third lateral lobe is small, being less than half as long, and scarcely two- 

 thirds as wide, as the second lateral, and provided with three nearly equal, 

 spreading, digitate, terminal bi'anches. Between the third lateral lobe and 

 the umbilical margin there are two other small, very unequal, lateral lobes, the 

 first of which has two or three digitations on each side; while the second is 

 nearly simple, or but slightly sinuous on the margins. 



In the number and arrangement of the lobes and sinuses of its septa, 

 as well as in their mode of branching, this species agrees very nearly with 

 .Phylloceras? Haiti, pi. 23, figs. 4 a, b, c (=Ammonites Haiti, Meek and 

 Hayden, Proceed. Acad. Nat. Sci. Phila.,* March, 1856, p. 70). It has, how- 

 ever, one more lateral branch on each side of its dorsal lobe, and one less on 

 each side of its superior lateral lobe, than ]*.! Haiti; while all the divisions 

 of its lobes and sinuses are more spreading Although rather closely allied 

 in their internal characters, these' two shells present marked differences in 

 form, as well as in their external markings; the species now under considera- 

 tion being much more ventricose and more coarsely ribbed than P.! Haiti. 



It agrees much more nearly in form with Ammonites Barnstoni, Meek 

 (Professor Hind's Report, Assiniboine and Saskatchewan Exploring Expedi- 

 tion, plate 11, figs. 1 and 2), from far up north, on Mackenzie's River, but 

 differs in having a smaller and more acutely conical umbilicus, and less 

 broadly-rounded periphery. Its costa? are also more rounded, and it presents 

 well-marked differences in its septa. 



The only specimen of this species yet seen consists entirely of septate 

 whorls, the non-septate outer portion having been broken away. It measures 

 in its greatest diameter 4 inches, and 2.57 inches in breadth at the widest 

 part of the outer whorl. It is scarcely necessary to add that this is fflot a 

 true Ammonite, ^as that genus is restricted to typical forms by the late 

 classifications. 



Named in honor of Lieut. John Mullan, United States Topographical 

 Engineers, in charge of the expedition for the location and construction of 

 the Pacific wagon-road. 



Locality and position. — Chippewa Point, near Fort Benton, on the 



Upper Missouri: Fort Benton group of the Upper Missouri Cretaceous 



series. 



77 II 



