432 Wbrtman — Studies of Eocene Mammalia in the 



ceous beds are extensively exposed in the same region in which 

 the Puerco fossils are found, and indeed the remains of many 

 characteristic Laramie land Vertebrates occur in the strata 

 immediately underlying those containing the Puerco, but not- 

 withstanding the most careful and extensive search, which I 

 have myself conducted, not a vestige of the Puerco forms has 

 ever been found in these underlying beds. The same is true of 

 the vast stretch of territory along the skirts of the Rocky 

 Mountain divide in which the Laramie is so extensively exposed. 

 Not a tooth, not a scrap nor a fragment of a bone referable to 

 these Puerco Eutherians has ever been found, in spite of the 

 fact that many of the most expert collectors have searched for 

 them assiduously in these, localities for many years. It is 

 hardly possible to explain their absence from the Cretaceous 

 on the basis of unfavorable conditions of preservation, for 

 the reason that we have not only the remains of many species 

 of land Vertebrates well and abundantly preserved, but we 

 have Mesozoic mammalian remains as well. The only explana- 

 tion which seems to me possible is that they were migrants 

 coming into these latitudes for the first time at the beginning 

 of the Puerco. 



The same argument applies with equal force to the new 

 elements which were introduced in the Torrejon, Wasatch, 

 Wind River, Bridger, and other epochs. They were simply 

 new-comers representing so many waves or impulses of migra- 

 tion, and the fact that practically the same species were intro- 

 duced at practically the same time, in the same order, and in 

 the same abrupt manner, in two such widely separated local- 

 ities as the Eastern and Western Hemispheres, adds over- 

 whelming proof that the two faunae had a common center of 

 dispersion. 



Now, where is it possible to locate such a land area common 

 to the two Hemispheres? Geologists have made out a suffi- 

 ciently complete history of the continental land masses, as 

 well as of the great ocean basins, to give a tolerably exact idea 

 of the main facts. All are agreed that at no time, at least 

 from the beginning of the Mesozoic to the present, were the 

 broad relations between the oceans and continents, either to 

 the east, west, or south of the localities under consideration, 

 materially different from what they are to-day. There is no 

 evidence whatever of any land connection between the two 

 Hemispheres at or near the Equator at any time in the past, so 

 that the only possible place where this common land area could 

 have existed was in the North. 



Let us examine the question again, from the standpoint of 

 the faunal changes which are indicated in the successive depos- 

 its of our American Tertiaries. In the Wind River and 



