Douglass : Merycochgerus. 93 



to Upper Miocene, yet the lists of species never precisely coincide 

 with those of other regions. 



In the three regions above mentioned, the plains, Oregon, and 

 Montana, it is a question how much the dissimilarities of the faunae 

 in these different localities are due to difference in time and how 

 much due to geographic distribution. Undoubtedly both are im- 

 portant factors. Personally I am more and more impressed with the 

 idea that only at long intervals have conditions in any one region 

 been favorable for the preservation of vertebrate fossils, and these 

 favorable local conditions may not have occurred at the same time in 

 widely separated localities, though evidently a great similarity of 

 conditions existed over a vast region during the deposition of the 

 Lower White River beds. 



It is evident that in the region in Colorado, which was studied by 

 Matthew, the upper strata of what he calls the White River are the 

 only representatives there of a long period of time during which 

 fossil-bearing deposits accumulated in Nebraska, Oregon, and Montana. 



I give a quotation which shows Dr. Matthew's views on the 

 subject : 



''The equivalence of the Titanotherium Beds and Oreodon Clays 

 with the corresponding horizons in South Dakota scarcely needs dis- 

 cussion, as the faunae are largely identical. The equivalence of the 

 Leptauchenia assise with the Protoceras sandstones is more difficult to 

 show, as the two have almost nothing in common. The Leptauchenia 

 clays of South Dakota, in the localities examined by W r ortman ^over- 

 lie the Protoceras sandstones ; but others have found them interbedded 

 and almost certainly contemporaneous. The uppermost levels of the 

 South Dakota clays, which no doubt are considerably above the sand- 

 stones, are said to be barren ; and in Colorado we found fossils scarce 

 in horizon C, but, when discovered, of much interest. They appear 

 to indicate that these comparatively barren upper clays are considerably 

 later than any of the more richly fossiliferous beds, and that the build- 

 ing up of the White River formation was continued into the Upper- 

 most Oligocene or Lower Miocene. For in the top levels we found 

 genera and even species hardly separable from those which occur in 

 the Loup Fork formation above, in company with the known Loup 

 Fork fauna, viz. : Merycochcerits proprhts, Anchippus texanus, Blasto- 



17 " On the Division of the White River or Lower Miocene of Dakota," Bull. 

 Amer. Mus. Nat. Hist., Vol V., p. 95. 



