FOSSIL VERTEBRA TES— MAMMALIA 835 
families specialized and diverged rapidly. The rhinocerotine 
differentiation in the broad sense of the term, imitated that of 
the Perissodactyla as a whole in its general functional radiation. 
They ran either into upland cursorial types, which competed 
with the Horses and the Ruminants, or into the lowland marsh 
or river dwellers, which competed with the Tapirs and the 
Titanotheres. 
Among the former were the smaller, more agile, light-chested 
types of Hyracodonts, simultating the Miocene Horses in skele- 
tal structure and in the development of true hoofs. Among the 
latter were the short, heavy types of Amynodonts, with broad, 
spreading, padded feet; they probably acquired, like the Tapirs, 
along, prehensile upper lip, or, possibly, a true proboscis was 
developed, in correlation with the rather abbreviated nasals. 
The elevated and prominent position of the orbit would bring 
the eye near the surface in swimming. This feature, with the 
long, curved tusks, undoubtedly used in uprooting, suggests 
the resemblance between the habits of these animals and those 
of the Hippopotami. The early Aceratheres were light-limbed 
rather swift-footed animals, intermediate in proportions between 
the Hyracodonts and the Amynodonts, but far less graceful 
and rapid than the former, yet the destiny of this family was 
also to finally produce both the very slow, heavy-bodied forms, 
such as Aceratherium (Aphelops) fossiger, of the Loup Fork and 
the stilted, long-limbed Aceratherium malacorhinum of the same 
period. 
Neither the Hyracodonts nor the Amynodonts developed 
horns, and all the early true Rhinoceroses had weak, hornless, 
nasals so that they probably appeared externally more like 
enlarged modern Tapirs than the well-armed animals we are 
now familiar with. 
They did not interfere with each other because each enjoyed 
a different local habitat, while occupying the general geographi- 
cal regions. The Hyracodonts dwelt in the drier grassy plains. 
The Amynodonts frequented the river and lake borders. Up to 
the time of the extinction of these two related families, the true 
