the Cretaceous-Tertiary Problem. 217 



Eiver mammals belong to the same families, but that the 

 Lance mammals are a more specialized and advanced 

 stage. This conforms to the evidence of the dinosaurs 

 and other reptiles, indicating that the Lance is consider- 

 ably later in time than the Belly River but that no great 

 migrational change in fauna occurred during that time. 



6. The researches upon the upper Cretaceous dino- 

 saurs by Brown, Lambe, Gilmore, Osborn and Parks, 

 have placed the correlation and succession of the later 

 dinosaur-bearing formations upon a very broad and solid 

 footing, so that it is hardly likely to be seriously ques- 

 tioned or materially modified. The exact relations of the 

 older phases of the Belly River and so-called Judith 

 River in southern Alberta and northern Montana have 

 been partly cleared up, but the complete results are not 

 yet available.^* 



Succession and Correlation of the Yertehrate Faunas as now 



understood. 



The Judith River formation of Montana containing 

 the Ceratops fauna is interbedded with the Fort Pierre 

 and its fauna is correlated with the Belly River. The 

 Milk River dinosaurs may be somewhat older. These 

 need hardly be discussed. The Ojo Alamo group in 

 New Mexico (Fruitland, Kirtland and Ojo Alamo for- 

 mations) contain a fauna correlated with the Judith and 

 Belly River. 



The Lance of Wyoming, Hell Creek of Montana, 

 Arapahoe and Denver of Colorado contain the Tricera- 

 tops fauna. The same six families of dinosaurs are 

 found, but some if not all are represented by more special- 

 ized genera and species. The same groups of turtles, 

 crocodiles and Rhynchocephalia are present, in some 

 cases at least with more specialized representatives. The 

 mammals (chiefly from the Lance) belong mostly, 

 probably all, to the metatherian families Plagiaula^idse 

 and Cimolestidae, but include more specialized genera 

 than are found in the Senonian. Detailed comparisons 

 indicate a very considerable lapse of time as measured 

 by the phyletic changes, but no great faunal break or 

 change, due to the invasion of a new fauna from 

 elsewhere. 



" See Brown, 1917, Bull. Am. Mus. Nat. Hist., 37, pp. 281-282. 



