FOSSIL HORSES IN AMERICA. 293 
the equine series, taken in succession from each of the geological 
periods in which this group of mammals is known to have lived. 
-The ancient Orohippus had all four digits of the fore feet well 
developed. In Miohippus, of the next period, the fifth toe has 
disappeared, or is only represented by a rudiment, and the limb is 
supported by the second, third and fourth, the middle one being 
the largest. Hipparion, of the later Tertiary, still has three digits, 
but the third is much stouter, and the outer ones have ceased to be 
of use, as they do not touch the ground. In Equus, the last of 
_the series, the lateral hoofs are gone, and the digits themselves 
are represented only by the rudimentary splint bones.* The 
middle, or third digit, supports the limb, and its size has increased 
accordingly. The corresponding changes in the posterior limb of 
these genera are very similar, but not so striking, as the oldest 
type (Orohippus) had but three toes behind. An earlier ancestor 
of the group, perhaps in the lowest Eocene, probably had four 
toes on this foot, and five in front. Such a predecessor is as 
clearly indicated by the feet of Orohippus, as the latter is by its 
Miocene relative. A still older ancestor, possibly in the Creta- 
ceous, doubtless had five toes in each foot, the typical number in 
mammals. This reduction in the number of toes may, perhaps, 
have been due to elevation of the region inhabited, which grad- 
ually led the animals to live on higher ground, instead of the soft 
lowlands where a polydactyl foot would be an advantage. 
The gradual elongation of the head and neck, which took place 
in the successive genera of this group during the Tertiary period, 
was a less fundamental change than that which resulted in the re- 
duction of the limbs. The process may be said to have already 
began in Orohippus, if we compare that form with other most 
nearly allied mammals. The diastema, or “ place for the bit,” was 
well developed in both jaws even then, but increased materially in 
succeeding genera. The number of the teeth remained the same 
until the Pliocene, when the front lower premolar was lost, and 
subsequently the corresponding upper tooth ceased to be function- 
ally developed. The next upper premolar, which in Orohippus 
was the smallest of the six posterior teeth, rapidly increased in 
size, and soon became, as in the horse, the largest of the series, 
The gr inding teeth at first had very short crowns, without cement, 
eet P S E 1 A nenaltie 
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on the fore foot. 
