SUCCESSIVE MAMMALIAN FAUNAS 265 



as the hoofed animals were concerned, and no member of any 

 of the six groups has ever been found outside of the Neotropical 

 region. 



4. Eocene 



North America. — In the western interior of North America 

 the Oligocene followed so gradually upon the Eocene, that there 

 is great difficulty in demarcating them and much difference 

 of opinion and practice obtains as to where the boundary fine 

 should be drawn. Not to depart too widely from the scheme 

 used by Professor Osborn, the Uinta stage is here treated as 

 uppermost Eocene, though this is a debatable procedure. 

 For several reasons, the extraordinarily interesting and sig- 

 nificant Uinta fauna is far less completely known than that of 

 the preceding Bridger and succeeding White River stages. For 

 one thing, it has been much less thoroughly explored, and it 

 may be confidently expected that future exploration will 

 greatly enlarge our knowledge. 



The smaller mammals of the Uinta are particularly ill- 

 known. No Insectivora have yet been found, though this 

 gap will assuredly be filled ; rodents are scanty in the collec- 

 tions and include only two famihes, one the fischyromyids, 

 which were still common in the White River, the other of 

 doubtful position, but not improbably to be considered as the 

 beginning of the pocket-gophers (Geomyidse). The archaic 

 flesh-eaters, or fCreodonta, were represented by two fami- 

 lies, one comprising smaller animals with somewhat cat-like, 

 shearing teeth ( fOxya^nidse) , the other, very large beasts 

 with crushing teeth (fMesonychidae), neither of which con- 

 tinued into the White River. As compared with the middle 

 and lower Eocene, the jcreodonts had greatly diminished and, 

 to replace them, the true Carnivora were beginning to come 

 in. As yet, however, only small and very primitive dog-like 

 forms are known and no trace of jsabre-tooths or mustelines 

 has been found. Indeed, it is very doubtful whether mem- 

 bers of these families ever will be found in the Uinta, for their 



