Rhinoceroses, 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 Amynodonts, 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 7’rigonias, 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 great 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 in 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 the 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 
equally gradual steps, take on the ruminant pattern. In the upper 
Miocene the line divides into the two branches of the camels and 
llamas, the former migrating to Eurasia and the latter to South 
America, though representatives of both lines persisted in North 
D. 13 
