CENOZOIC ERA: AGE OF MAMMALS 619 



adapted to the mastication of plains vegetation. 



Various side branches appeared and became 



extinct during the period. Among them one 



unusual form (Protoceras) which was about 



the height of a sheep had two pairs of short, 



bony horns and canine tusks (Fig. cc*). Its , FlG ; ? 53- ~ A 7 four *" 



, . . , ,1 , t horned deer (Syndyoce- 



ancestry is unknown, and it probably reached ras ) (Upper Oligocene). 



North America by immigration. (After H. F. Osborn.) 



REFERENCES FOR DEER 



Matthew, W. D., — Osteology of Blastomeryx and Phylogeny of the American Cervida*: 



Bull. Am. Mus. Nat. Hist., Vol. 24, 1908, pp. 535-562. 

 Romanes, G. J., — Darwin and after Darwin, Vol. 1, pp. 166-169. 

 Woodward, A. S.,. — Vertebrate Paleontology, pp. 363-365. 



Cattle, Sheep, and Goats. — True cattle first appear, as far as 

 known, in the early Pliocene deposits of Asia. Concerning the 

 geological history of cattle, sheep, and goats little is known, since 

 the difference in the ox, antelope, sheep, and goats is largely a matter 

 of the curve of the horn and the build. This entire family (Bovidae) 

 have their maximum development at the present time. 



Swine and Related Animals. — True pigs were confined to the 

 Old World until brought to the New by Europeans, although their 

 relatives, the peccaries, were abundant in portions of both North 

 and South America. The entire family is very simple in structure, 

 being the least altered of the descendants of the early, even-toed 

 mammals (artiodactyls), and dates back to the early Eocene, although 

 the oldest species of the true pig is not known before the Miocene. 

 Several extinct families, distantly related to the pig, played an im- 

 portant part in the life of the Tertiary; and of these one (Entelodon) 

 is especially worthy of mention. One species was as large as a 

 rhinoceros, with a head four feet long. One peculiarity of the skull 

 consisted in a prolongation of the cheek bones on either side of the 

 lower jaw. They had two-toed feet, being rather highly specialized 

 in this and other particulars. 



Another generalized animal (Oreodon), in some respects combining the characters 

 of the deer and hog but not ancestral to them, was extremely abundant at certain 

 times during the Oligocene and Miocene and may have been partially responsible for 

 the extinction of the titanotheres. It was not larger than a sheep, with a long tail 

 and four-toed feet. 



A Climbing Ungulate. — An interesting example of a modification in structure, 

 fitting an animal for conditions very different from those to which its relatives were 



