1916] Taylor: Beavers of Western North America 459 



all earlier formations of the latter continent except for the one 

 instance mentioned above, the correlation of the formation in which 

 the tooth was found being still somewhat uncertain. 



If there has not been independent origin of Steneofiber, Eucastor- 

 Dipoides, and Castor itself on the Eurasian and North American 

 continents, it is clear that there have been several intercontinental 

 migrations of beavers. Although we are not able to state exactly 

 the number of these migrations, the evidence indicates that there 

 were at least three. 



"While it is realized that negative evidence is likely to prove 

 untrustworthy, and that possible errors in correlation introduce a 

 further element of uncertainty into general statements as to the 

 origin and migration of the castorids, insofar at least as these are 

 based on relative time of appearance, it is believed that the following 

 tentative propositions merit consideration. 



Steneofiber probably developed first in the Old World, since it is 

 found in Europe in Middle Oligocene while it does not appear in 

 North America until the Upper Oligocene. By late Oligocene time, 

 therefore, its migration had apparently carried it into North America. 



The fact that the John Day epoch of the Oligocene is marked by 

 the disappearance of almost all the European migrants which are so 

 characteristic of the earlier White River fauna, with other evidence, 

 has seemed to show that the land connection with the Old Woi-ld was 

 broken (Scott, 1913, p. 116). The case of Steneofiber might be taken 

 to indicate, though such an isolated instance is probably not worth 

 much, that the White River land connection was maintained into 

 the early part of the John Day phase of the Upper Oligocene. It is 

 more probable that Steneofiber crossed the connecting land bridge 

 during White River time, but did not attain a widespread distribu- 

 tion until the John Day epoch. 



The E ucastor-Dipoides stock would seem to have taken origin in 

 North America, since it is first found in the Upper Miocene of that 

 continent. By Pliocene time it had migrated at least into Asia. It 

 seems certain that Bering Strait was closed during at least a great 

 part of that epoch (Scott, 1913, p. 125). 



The genus Castor probably arose in Eurasia from Steneofiber or 

 a closely related stock. A wave of migration carried it into North 

 America during the Pliocene, probably by way of the North-Pacific 

 land-connection, and its remains became widely distributed and fairly 

 abundant in the Pleistocene. 



