396 AN INTRODUCTION TO THE STUDY OF FOSSILS 



high-crowned, usually without roots, and cement-covered. This 

 type of tooth is well fitted for grazing, for the soft cement and 

 hard enamel are folded down into the dentine, which is inter- 

 mediate in hardness, and consequently the grinding surface is 

 always rough from the projecting enamel ridges (compare with 

 Fig. 168). During the Pliocene and Pleistocene (Fig. 164, 6) 

 Equus migrated from America into all other continents except 

 Australia, over the land connections then existing. It had, how- 

 ever, become extinct in North and South America before the 

 time of Columbus ; all wild horses here are escaped descendants 

 of those brought in by European explorers. 



The evolution of the horse thus occurred upon the North 

 American continent in the era immediately succeeding its ele- 

 vation from the Cretaceous seas. At the beginning of this 

 Cenozoic Era the warped land surface was necessarily occupied 

 by many swamps, lakes and aggrading stream beds. Later, the 

 filling up of these depressions, accompanied by the growing 

 aridity produced by the increasing elevation of the land to 

 mountain heights, led to the drying up of many water courses 

 and to the necessity of greater speed in the inhabitants to reach 

 food and water. The incoming of the more siliceous grasses 

 about the middle of this era, which adapted themselves to this 

 more arid climate, placed a premium also upon longer and better 

 grinding teeth. 



(9) Ancylopoda (Eocene to Pliocene), a primitive sub-order 

 closely related to the Perissodactyla, and widely distributed 

 over most of the world. 



(10) The Artiodactyla (Lower Eocene to present). In these, 

 the even-toed ungulates, the third and fourth toes of both fore and 

 hind feet are the longest and are of equal size (whence the name 

 from Greek artios, even, + dactylos, a finger). The plane of 

 symmetry of the foot passes between the third and fourth digits. 

 In the earlier forms the grinding teeth are low-crowned and 

 tuberculate ; later (in cattle and antelopes) these tubercles unite 

 to form continuous crescents, and the crown becomes almost as 



