CHORDATA — VERTEBRATA — MAMMALS 397 



high as in the horses among the Perissodactyla. Pigs, dat- 

 ing from the Middle Eocene, are the least modified descendants 

 of the early Artiodactyla ; even the canine teeth are pretty 

 well retained. True pigs (Sus), known from the Miocene 

 to present, had, up to the time of Columbus, always been con- 

 fined to Eurasia and Africa. Another very primitive family, 

 that of the Hippopotami, now confined to Africa but formerly 

 found throughout Eurasia and Africa, has thus far been traced 

 back only to the Upper Miocene. 



It is likely that all lower Tertiary Artiodactyla, like the prim- 

 itive existing forms noted above, possessed a simple stomach 

 and did not chew the cud ; this power was probably accompanied 

 by the development of the crescent-shaped ridges, so character- 

 istic of modern ruminants. An example of primitive, possibly 

 transitional, ruminants is seen in the extinct family of the oreo- 

 donts ranging in time from the Upper Eocene to the Lower 

 Pliocene and confined to North America ; these were very 

 numerous, not larger than a sheep, with four functional toes 

 upon each foot and a very long tail ; the genus Oreodon is con- 

 fined to the Oligocene. The family of camels (camels, llamas, 

 etc.) was evolved upon the continent of North America from 

 primitive Upper Eocene forms ; they were very abundant here 

 during the Oligocene and Miocene. During the Pliocene they 

 migrated into South America, Asia and Africa, where they exist 

 to the present, but they disappeared from their native home 

 during the Pleistocene. 



In the following forms, — the true ruminants, the upper 

 incisors are always absent, and likewise usually the upper canines. 

 North America played a very insignificant role in the evolution 

 of these forms. In the solid-horned ruminants (giraffes and 

 deer) the usually branched horn, an outgrowth of the frontal 

 bone, is shed each year ; it usually increases in size and number 

 of branches with each renewal. The giraffes (Pliocene to pres- 

 ent), now confined to Africa, were formerly present also in 

 Eurasia; they never had any representatives in the Western 



