546 E. RUSSELL LLOYD AND C. J. HARES 



fauna lived over from the Cretaceous into the Tertiary and thus 

 became contemporaneous with the totally different marine faunas 

 of the Gulf and Atlantic coasts, but if this did happen it is the first 

 recorded case of such contemporaneous Cretaceous and Eocene 

 faunas. 



The evidence of the reptilian fossils in the Lance formation 

 supports the evidence of the marine fauna. This — the Triceratops 

 fauna — is the last of the great dinosaur faunas of the Mesozoic 

 and for that reason has been considered generally by vertebrate 

 paleontologists as sufficient evidence of the Mesozoic age of the 

 formation. It is, however, just as probable that the Ceratopsian 

 dinosaurs lived over into the Tertiary as that the marine Fox Hills 

 fauna did so. 



There remains yet to be considered the evidence of the mam- 

 malian fauna of the Fort Union formation. This fauna, of course, 

 has its closest affinities with the succeeding Eocene faunas, since 

 practically no mammalian fauna is known in the later Cretaceous 

 formations. There is here the possibility that the Tertiary mam- 

 malian faunas began their development before the close of the 

 Cretaceous. Lee'' has recently described a find of a bone of a 

 Creodont mammal associated with Ceratopsian bones in the 

 Dawson arkose which shows that Ceratopsian dinosaurs and 

 mammals of Tertiary aspect must have lived at the same 

 time. 



Such in outline is the Lance problem as it stands today. A 

 recent detailed consideration of all of the evidence has led to a 

 decision by the U.S. Geological Survey that the Denver and 

 Arapahoe, Dawson, and Raton formations in Colorado and 

 New Mexico all be placed in the Tertiary system. This deci- 

 sion was based primarily on the correlation of these formations 

 with the Wilcox formation of the Gulf region on the evidence of 

 their fossil floras and also on the consequent correlation of the 

 unconformities in the two regions. Although the Lance forma- 

 tion is believed to be of about the same age as the Denver, Raton, 

 and "Upper Laramie," it is classified by the U.S. Geological Survey 



' W. T. Lee, "Recent Discovery of Dinosaurs in the Tertiary," Am. Jour. Sci., 

 4th ser., XXXV (1913), 531-34- 



