FOSSIL HORSES IN AMERICA. 289 
developed in both sexes, and the incisors did not have the “mark” 
which indicates the age of the modern horse. The radius and 
ulna were separate, and the latter was entire 
throughout its whole length. The tibia and fibula 
were distinct. In the fore foot, all the digits ex- 
cept the pollex, or first, were well developed, as 
shown in the accompanying figure (73) of the left 
fore foot of Orohippus agilis Marsh. The third 
digit is the largest, and its close resemblance to 
that of the horse is clearly marked. The terminal y 
phalanx, or coffin bone, has a shallow median 
groove in front, as in many species of this group x 
in the later Tertiary. The fourth digit exceeds Drok 
the second in size, and the fifth is much the shortest 
of all. Its metacarpal bone is considerably curved outward. In 
the hind foot of this genus, there are but three digits. The fourth 
metatarsal is much larger than the second. ; 
The only species of Orohippus at present known are from the 
Eocene of Wyoming and Utah, and are as follows :—Orohippus 
gracilis Marsh, O. pumilus Marsh, O. agilis Marsh, and O. major 
Marsh.* 
In the middle Tertiary, or Miocene, there were two other lakes 
on either side of the great Eocene basin. The largest of these 
was east of the Rocky Mountains, extending over portions of 
what are now Dakota, Nebraska and Colorado. The clays depos- 
ited in this lake form the “ Mauvaises terres,” or “ Bad lands,” of 
that region, and are well known for their fossil treasures. The 
other Miocene lake was west of the Blue mountains, where eastern 
Oregon now is, but its extent is unknown, as this whole region 
has since been covered with a vast sheet of basalt, a thousand or 
more feet in thickness, and the original lake sediments are only to 
be seen where this lava has been washed away. In both of these 
ancient lake basins, many remains of animals allied to the horse 
are found, showing that during the Miocene this group of mam- 
mals were well represented. 
In the western, or Oregon basin, the genus Miohippus Marsh 
first makes its appearance. It resembles Orohippus of the 
Eocene in its general characters, especially in the shape of the 
skull, number and form of teeth, and separate ulna; but it had 
* American Journal of Science, Vol. vii, p. 247, March, 1874. 
AMER. NATURALIST, VOL. VIII. 19 
| Fig. 73. 
N 
y 
