136 DISTRIBUTION OF EXTINCT ANIMALS. [PART II. 
Utah and Wyoming, we get a step further back, several species 
having been discovered about the size of a fox with four toes in 
front and three behind. These form the genus Orohippus, and 
are the oldest ancestral horse known. Prof. Marsh points out the 
remarkably perfect series of forms in America, which, beginning 
with this minute ancient type, is gradually modified by gaining 
increased size, increased speed by concentration of the limb-bones, 
elongation of the head and neck, the canine teeth decreased in 
size, the molars becoming longer and being coated with cement— 
till we at last come to animals hardly distinguishable, specifically, 
from the living horse. 
Allied to these, area series of forms showing a transition to the 
tapirs, and to the Paleotheriwm of the European Eocene. In the 
Pliocene we have Parahippus ; in the Miocene Lophiodon, found 
in the same formation and in the Eocene of Europe, and allied 
to the tapir; and in the Eocene, Palwosyops, as large as a rhino- 
ceros, which had large canines and was allied to the tapir and 
Paleotherium ; Limnohyus, forming the type of a family Limno- 
hyide, which included the last genus and some others mentioned 
further on; and Hyrachyus, allied to Lophiodon, and to Hyracodon 
an extinct form of rhinoceros. Besides these we have Lophiothe- 
rium (also from the Eocene of Europe); Diplacodon allied to 
Limnohyus, but with affinities to modern Perissodactyla and nearly 
as large as a rhinoceros; and Colonoceras, also belonging to the 
Limnohyide, an animal which was the size of a sheep, and had 
divergent protuberances or horns on its nose. A remarkable 
genus, Bathmodon, lately described by Professor Cope, and of 
which five species have been found in the Eocene of New Mexico 
and Wyoming, is believed to form the type of a new family, 
having some affinity to Palwosyops and to the extinct Bronto- 
theride. It had large canine tusks but no horns. 
The Rhinocerotide are represented in America by the genus 
Rhinoceros in the Pliocene and Miocene, and by Acerathervwm 
and Hyracodon in the Miocene. Both the latter were hornless, 
and Hyracodon was allied to the Eocene Hyrachyus, one of the 
Lophiodontide. In the Eocene and Miocene deposits of Utah, 
and Oregon, several remarkable extinct rhinoceroses have been 
