254 LAND MAMMALS IN THE WESTERN HEMISPHERE 



were migrants from the eastern hemisphere, for, though small 

 and primitive representatives of it occurred in the North 

 American Eocene, as well as in the corresponding formations 

 of Europe, the family appears to have died out in America 

 and to have been renewed by the Oligocene migration. 



Coincident with this decline of the fcreodonts and, no doubt, 

 causally connected with it, was the rise of the true Carnivora, 

 which for the first time were numerous and were divisible into 

 three distinct famiUes. Small and primitive representatives 

 of the wolves {'\Daphoenus) and possibly also of the foxes 

 {^Cynodictis) were quite common, and there were a few species 

 of the musteline family, evidently immigrants and the most 

 ancient yet found in America. There were several species of 

 the jsabre-tooth cats {^Dinictis and ^Hoplophoneus) all of 

 which, except in the uppermost substage, were quite small, 

 few of them exceeding the Canada Lynx in size. A much 

 larger animal {^Eusmilus, also European) appeared in the 

 latter part of the stage. None of the true cats, or feline sub- 

 family, has been obtained. Nothing is yet known of the time 

 and place of origin of the fsabre-tooth series, for they ap- 

 peared at approximately the same date in Europe and America, 

 and in neither continent have any possible ancestors been found 

 in preceding formations. The problem is like that of the 

 Proboscidea (see p. 234), but Egypt has given no help in the 

 case of the fsabre-tooths, and, by a process of elimination, 

 we reach the conclusion that these strange creatures probably 

 arose somewhere in Asia and sent out migrants eastward and 

 westward. 



The Rodentia were fairly abundant and present a strange 

 mixture of ancient and comparatively modern types. One 

 very common genus {\Ischyromys), which was the last rem- 

 nant of a family almost limited to the North American 

 Eocene, was associated with the earliest American mice, 

 arboreal and ground squirrels, beavers and rabbits; some, if 

 not all, of these were immigrants. 



