120 LAND MAMMALS IN THE WESTERN HEMISPHERE 



is shown by the appearance of Old World land-mammals, 

 beginning scantily in the lower and becoming numerous in 

 the middle Miocene. This connection, it will be remembered, 

 had been interrupted during the upper Oligocene. Many 

 students of the problem have maintained that the land-bridge 

 was by way of the West Indies and the Mediterranean lands, 

 but such a bridge would not account for the facts of mammalian 

 distribution, which would seem to require its location in the 

 far north. 



Several distinct lines of evidence go to prove that the 

 junction of the Americas dates from the Miocene, possibly 

 from the beginning of it. The absence of Atlantic species 

 from the Pacific Miocene is an indication that the passage 

 from ocean to ocean had been closed, and this is confirmed by 

 the geology of the Central American and Isthmian region. 

 In the middle Miocene of Oregon and Nebraska have been 

 found remains, which are unfortunately too incomplete for 

 altogether convincing identification, but which can be inter- 

 preted only as belonging to the extinct and most characteristi- 

 cally South American group of edentates, the tground-sloths or 

 fGravigrada ; if this reference is correct, the fact of the junc- 

 tion cannot be questioned. 



Continental deposits of Miocene date, chiefly accumula- 

 tions made by rivers and the wind, cover vast areas of the west- 

 ern interior, though but rarely to any considerable depth. 

 These have been divided into several stages and have received 

 various names ; the lower Miocene, known as the Arikaree, 

 Harrison or Rosebud, overlies the White River in South Dakota, 

 western Nebraska and eastern Wyoming, with smaller areas in 

 Montana and Colorado. In the deposits of this stage there 

 are no mammals of indisputably Old World type, though a few 

 which I consider to be such are a probable indication of re- 

 newed connection with Europe. The middle Miocene is 

 found typically in central Montana, where it is called the 



t Extinct. 



