O. C. Marsh— Vertebrate Life in America. 859 
the premolars. The ulna and the fibula were entire and dis- 
tinct, and there were four well developed toes and a rudiment 
of another on the fore feet, and three toes behind. In the 
structure of the feet, and in the teeth, the ippus indicates 
unmistakably that the direct ancestral line to the modern horse 
has already separated from the other Perissodactyles. In the 
next higher division of the Eocene, another genus (Orohippus) 
makes its appearance, replacing Hohippus, and showing a 
greater, although still distant, resemblance to the Equine type. 
The rudimentary first digit of the fore foot has disappeared, 
and the last premolar has gone over to the molar series. ‘0- 
hippus was but little larger than Hohi ypus, and in most other 
respects very similar. Several species have been found in the 
same horizon with Dinoceras, and others lived during the upper 
Kocene with Diplacodon, but none later. 
ear the base of the Miocene, in the Brontotherium beds, 
we find a third closely allied genus, J/esohippus, which is about 
as large as a sheep, and one stage nearer the horse. There are 
only three toes and a rudimentary splint bone on the fore feet, 
and three toes behind. Two of the premolar teeth are quite 
like the molars. The ulna is no longer distinct, or the fibula 
entire, and other characters show clearly that the transition is 
advancing. In the upper Miocene, Mesohippus is not found, 
but in its place a fourth form, Afiohippus, continues the line. 
This genus is near the Anchitherium of Kurope, but presents 
several important differences. The three toes in each foot are 
more nearly of a size, and arudiment of the fifth metacarpal 
bone is retained. All the known species of this genus are larger 
than those of Aesohippus, and none pass above the Miocene. 
The genus Protohippus of the lower Pliocene, is yet more 
equine, and some of its species equaled the ass in size. There 
are still three toes on each foot, but only the middle one, cor- 
responding to the single toe of the horse, comes to the ground. 
This genus resembles most nearly the Hipparion of Europe. 
In the Pliocene, we have the last stage of the series before 
reaching the horse, in the genus Pliohippus, which has lost the 
small hooflets, and in other respects is very equine. Only in 
the upper Pliocene, does the true Aguus appear, and complete 
the genealogy of the Horse, which in the Post-Tertiary roamed 
over the whole of North and South America, and soon after 
became extinct. This occurred long before the discovery of 
the Continent by Europeans, and no satisfactory reason for the 
extinction has yet been given. Besides the characters I have 
mentioned, there are many others, in the skeleton, skull, teeth, 
and brain of the forty or more intermediate species, whic 
show that the transition from the Eocene Hohippus to the 
modern Hguus, has taken place in the order indicated, and I 
