214 



THE AGE OF MAMMALS 



to the Old World form. The light-limbed machacrodont, or saber-tooth 

 cats (Dinictis) are characteristic, as well as the first mustelids (Buncelurus) 

 known in this country. 



Every division of the mammals seems to have differentiated into its 

 plains-living and open country types and forest- and river-living types. 



Fic. lU- 



Tlic Lower Oligocene cursorial rhinoceros Hyracodon. Alter unguuil by Charles 

 R. Knight in the American Museum of Natural History. 



Of the former, we observe, among the rodents, the leporids or hares; 

 among rhinoceroses, the light-limbed hyracodonts; among the lophiodonts, 

 Colodon. The horses of the period are still polyphyletic, — small, exces- 

 sively light-limbed, swift animals, models of grace and beauty. Among 

 carnivores, both the canids and machserodont cats are partly cursorial. 

 The scarcely known camels were also plains-living types, although still 

 brachyodont. The peccaries (Dicotylidae) first appear here. 



Of the contrasting forest and lowland fauna, among perissodactyls 

 may be cited the titanotheres, found in the Swift Current Creek deposits 

 of British Columbia. The forest-living tapirs are not known. Among 

 artiodactyls, Agriochm-us, a genus of oreodont, also the anthracothere 

 Ancodus are probably river-border or forest animals. The amynodont 

 rhinoceroses now take on a distinctly fluviatile, or river-living type; their 

 remains are found only in the river-channel sandstones. Most of the 

 titanotheres were browsers and frequented river borders in the lower plains. 



