FOSSIL HORSES IN AMERICA. 291 
of Leidy, to whose researches we are so largely indebted for 
our present knowledge of this group. Of these Pliocene genera, 
more than twenty species have been described from American 
strata, all apparently larger than their Miocene relatives, but all 
smaller than the present horse, and many of them approaching 
the ass in size. Among the more characteristic of these species 
may be mentioned, Anchippus Texanus Leidy, from Texas; A. 
brevidens Marsh, from Oregon; Hipparion occidentale Leidy, and 
H. speciosum Leidy, from Nebraska; Protohippus perditus Leidy, 
from the Niobrara; P. parvulus Marsh, from Nebraska, the 
smallest Pliocene species; Parahippus cognatus Leidy, and Plio- 
hippus pernix Marsh, from the Niobrara. 
In the upper Pliocene, or more probably in the transition beds 
above, there first appears a true Equus, and in the Quaternary de- 
posits, remains of this genus are not uncommon. Five or six 
species are known from the United States, and several others from 
Central and South America. The latest extinct species appears 
to have been Equus fraternus Leidy, which cannot be distinguished 
anatomically from the existing horse. These later extinct horses 
are all larger than the Pliocene Equines, and some of them even 
exceeded in size the living species. 
The large number of equine mammals now known from the 
Tertiary deposits of this country, and their regular distribution 
through the subdivisions of this formation, afford a good opportu- 
nity to ascertain the probable lineal descent of the modern horse. 
The American representative of the latter is the extinct Equus 
Sraternus Leidy, a species almost, if not entirely, identical with 
the old world Equus caballus Linn., to which our recent horse be- 
longs. Huxley has traced successfully the later genealogy of the 
horse through European extinct forms,* but the line in America 
was probably a more direct one, and the record is more complete. 
aking, then, as the extremes of a series, Orohippus agilis Marsh, 
from the Eocene, and Equus fraternus Leidy, from the Quaternary, 
intermediate forms may be intercalated with considerable cer- 
tainty from the thirty or more well marked species that lived 
in the intervening periods. The natural line of descent would 
seem to be through the following genera:— Orohippus, of the 
Eocene ; Miohippus and Anchitherium, of the Miocene ; Anchippus, 
* Anniversary Address, Geological Society of London, 1870. 
