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invariably smaller than the frontal horns, but coupled with the smaller size and open 

 temporal fossae, it would appear to be more primitive. 



The new genus Stegoceras, proposed in this memoir, may represent a type with small 

 nasal horns, as in some of the Laramie Ceratopsids, such as Slerrholophus. 



It is not at all improbable that the horned dinosaurs will prove to be diphyletic, one 

 line with persistent open fossse leading from Monoclonius to Torosaurua, the other leading 

 to Triceratops with closed fossae. 



MAMMAI,IA. 



Of the two Mammals discovered in the Belly Eiver, Ptilodus primwvus, judging by 

 the condition of the grooves upon its premolars and tubercles upon its molar teeth, is 

 undoubtedly more primitive than the Laramie plagiaulacids. 



5. GrENERAL CONCLUSION. 



The conclusion is that the Belly River fauna is more ancient in character both as to 

 the older types of animals which it contains and as to the stages of evolution amono- 

 animals which are also represented in the Laramie. The geological interval represented 

 by the Ft. Pierre-Fox Hills marine beds was accompanied by the extinction of certain 

 Jurassic types and progressive evolution of the persistent types. Finally the tossil 

 vertebrates hitherto described from Montana probably are, in part at least, of Mid-Cretace- 

 ous or Belly Eiver age. 



Amekican Museum of Natural History, 

 July 25, 1902. 



