OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 239 



the modern ruminants, and the lophoselenodont, characterizing the 

 horses. 



Titanothere grinding teeth. — We have seen the magnificent titanotheres 

 suddenly cut off at the close of Lower Oligocene times, and this extinction 

 may bo attributed partly to the cone-and-crescent, or bunoselenodont 



Hippopotami Anthracotheres Oreodonts Hyracodonts 

 Pi§s, Peccaries Anoplotheres Hypertra^ulids Rhinoceroses 

 Achaenodonts Cfnotheres, Lophiodonts 



Elotheres Xiphodonts Camels 



True Ruminants 



Horses Titanotheres 

 Hyracotheres Chalicotheres 



Artio dactyla 



Pe rissodactyla 



EVOLUTION OF UNGULATES IN NORTH AMERICA 



Fig. 122. — The sur\aval or extinction of mammals possessing certain types of molar teeth. 



pattern of their grincUng teeth, which were adapted to browsing on the 

 coarse and soft rather than grazing upon the fine and hard kinds of food. 

 Among these animals, nature was apparently making an effort to convert 

 a brachyodont into a hypsodont cro"\m by the elongation of the outer side 

 of the superior grinding teeth; l)ut this effort was futile because of the 

 absence of a cross-crest and the persistent brachyodonty of the inner 



