ON THE EXTINCT ANIMALS OF NORTH AMERICA. 
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During this period an immense development took place in 
the various forms of three-toed horses, Anchippus , Proto- 
hippus , Parahippus , Hipparion , &c., which replaced the 
Anchitherium of the Miocene. These in their turn, through 
many well-marked gradational forms (a full knowledge of 
which is among the many interesting results of these explora- 
tions), gave way to the true horses, of which remains of several 
species have been found in Pleistocene deposits, scattered 
throughout almost every region of the continent from Escholtz 
Bay in the north to Patagonia in the south. These also be- 
came entirely extinct before the discovery of America by the 
Spaniards — a most remarkable circumstance, considering the 
fitness of the country for their maintenance, as proved by the 
facility with which the descendants of horses introduced by 
European invaders have multiplied in a feral state. 
On the other hand, of tapirs, fossil remains have been found 
most sparingly, though sufficient to show that they had a much 
wider geographical range northwards in the Pleistocene period 
than now, and yet several species of this genus still linger in 
the highlands of Central and South America, the sole direct 
representatives of the vast and varied Perissodactyle fauna of 
the continent in ages long gone by. 
We may now pass to the remaining great group of hoofed 
animals, the even-toed or Artiodactyles, represented at present 
by the pigs, hippotami, camels, chevrotains, deer, antelopes, 
sheep, and oxen. 
The remains of this group in the hitherto explored American 
Eocenes are very scanty and unsatisfactory as affording indica- 
tions of its early history and development. Not a single speci- 
men has yet been described which was found in a sufficiently 
perfect state of preservation to give a tolerably correct idea of 
its structure and affinities, and no forms corresponding to the 
well-established European Anoplotherium , Dichoclon , Xipho- 
don , or Ccenotherium have been found. Towards the close of 
the period only (if the age of the Tertiary beds of Utah are 
rightly determined) do we find indications of well-defined 
crescentic-toothed or Selenodont species ( Agriochcerus , Leidy), 
and also of tubercular-toothed or Bunodont forms ( Elotherium 
and Platygonus). During the Miocene period, however, 
Artiodactyles of both these two main divisions abounded. It 
will be as well to take each group separately, and follow its 
history throughout, from the Miocene to the present day. 
1 . The Bunodonts, or those which inclined most to the pigs 
in dental structure. These were in North America, as in 
Europe, chiefly represented by animals of the genus Elotherium , 
huge swine-like creatures, some of whom approached the hippo- 
potamus in size, and also by an allied four-toed form, Pelonax 
