294 FOSSIL HORSES IN AMERICA, 
and were inserted by distinct roots. In Pliocene species, the — 
molars became longer, and were more or less coated with cement. ; 
The modern horse has extremely long grinders, without true roots, 
and covered with a thick external layer of cement. The canine 
teeth were very large in Orohippus, and in this genus, as well as 
those from the Middle Tertiary, appear to have been well devel — 
oped in both sexes. In later forms, these teéth declined in size, 
especially as the changes in the limbs afforded other facilities for 
defence, or escape from danger. The incisors in the early forms — 
were small, and without the characteristic “mark” of the modem — 
horse. In the genera from the American Eocene and Miocene, — 
the orbit was not enclosed behind by an entire bridge of bone, 
and this first makes its appearance in this country in Pliocene. 
forms. The depression in front of the orbit, so characteristic of — 
Anchitherium and some of the Pliocene genera, is, strange to sayy : 
not seen in Orohippus, or the later Miohippus, and is wanting, 
likewise, in existing horses. It is an interesting fact that the pê- 
culiarly equine features acquired by Orohippus are retained per — 
sistently throughout the entire series of succeeding forms. Such, — 
e. g., is the form of the anterior part of the lower jaw, and also | 
the characteristic astragalus, with its narrow, oblique, superior : 
ridges, and its small articular facet for the cuboid. a 
Such is, in brief, a general outline of the more marked changes 2 
that seem to have produced in America the highly specialized i 
modern Equus from his diminutive, four-toed predecessor, m 
Eocene Orohippus. The line of descent appears to have hem w 
rect, and the remains now known supply every important inti 
mediate form. It is, of course, impossible to say with certaint 
through which of the three-toed genera of the Pliocene that li ey 
together, the succession came. It is not impossible that the Lg 
species, which appear generically identical, are the descendants 
more distinct Pliocene types, as the persistent tendency in all ye 
earlier forms was in the same direction. Considering the ren 
able development of the group through the entire Tertiary a 
and its existence even later, it seems very strange that none oft : 
species should have survived, and that we are indebted for w 
present horse to the old world. ' n 
