Rliinoceroses, Camels, and Llamas 193 



the contemporary Oligocene horses, and was evidently adapted for 

 speed. It may well have been the competition of the horses which 

 led to the extinction of these cursorial rhinoceroses. 



The second sub-family, that of the Am}Tiodont3, followed a 

 totally different course of development, becoming short-legged and 

 short-footed, massive animals, the proportions of which suggest 

 aquatic habits; they retained four digits in the front foot. The 

 animal was well provided with weapons in the large canine tusks, 

 but was without horns. Some members of this group extended 

 their range to the Old World, but they all died out in the middle 

 Oligocene, leaving no successors. 



The sub-family of the true rhinoceroses cannot yet be certainly 

 traced farther back than to the base of the middle Oligocene, though 

 some fragmentary remains found in the lower Oligocene are probably 

 also referable to it. The most ancient and most primitive member of 

 this series yet discovered, the genus Trigonias, is unmistakably a 

 rhinoceros, yet much less massive, having more the proportions of a 

 tapir ; it had four toes in the front foot, three in the hind, and had a 

 full complement of teeth, except for the lower canines, though the 

 upper canines are about to disappear, and the peculiar modification 

 of the incisors, characteristic of the true rhinoceroses, is already 

 apparent ; the skull is hornless. Representatives of this sub-family 

 continue through the Oligocene and Miocene of North America, 

 becoming rare and localised in the Pliocene and then disappearing 

 altogether. In the Old World, on the other hand, where the line 

 appeared almost as early as it did in America, this group underwent 

 a gi'eat expansion and ramification, giving rise not only to the 

 Asiatic and African forms, but also to several extinct series. 



Turning now to the Artiodactyla, we find still another group of 

 mammals, that of the camels and llamas, which has long vanished 

 from North America, yet took its rise and ran the greater part of its 

 course in that continent. From the lower Eocene onward the history 

 of this series is substantially complete, though much remains to be 

 learned concerning the earlier members of the family. The story is 

 very like that of the horses, to which iu many respects it runs 

 curiously parallel. Beginning with very small, five-toed animals, we 

 observe in the successive genera a gradual transformation in all parts 

 of tlie skeleton, an elongation of the neck, limbs and feet, a reduction 

 of the digits from five to two, and eventually the coalescence of the 

 remaining two digits into a " cannon-bone." The grinding teeth, by 

 e<[ually giadual steps, take on the ruminant pattern. In the upper 

 Miocene the line divides into the two branclies of the camels and 

 llamas, the former migrating to Eurasia an<l the latter to South 

 America, though representatives of both lines persisted in North 



D. 13 



