248 FROM OROATPPUS TO MIOHIPPUS CHAP. 
has a well-marked coracoid process. The radius and ulna are 
separate; so too are the tibia and fibula. Hohippus, belonging 
to the same sub-family, is shghtly more primitive; for the hind- 
feet have a rudiment of digit I. Ovrohippus is a little nearer to 
the Horses in that the molar teeth have acquired a little further 
advance towards the equine type. Instead of the tubercles of the 
teeth remaining for the most part separate, they have fused into a 
set of ridges, of which, however, the pattern is less complex than 
in the modern Horses. In other respects Orohippus is much like 
Hyracotherium.  Pachynolophus seems to be but a synonym. 
The next stage is shown by JMesohippus, a Lower Miocene 
form, usually referred to the neighbourhood of Palaeotherium. It 
has nearly lost one of the toes of the fore-foot, a rudiment only 
remaining ; the metapodials, at any rate of the fore-feet, seem to 
be slightly increased in length. The orbit is not encircled by 
bone, but there is a strong process from the frontal, which nearly 
meets the zygomatic arch. 
Anchitherium, from the Upper Miocene, is not far removed in 
structure from the last-mentioned form; it is a trifle nearer the 
existing Horse in several points. The ulna is further reduced and 
fused with the radius below: the rudiment of digit V is still 
more rudimentary ; the two lateral digits are smaller in proportion 
to the central one than they are in Mesohippus ; the fibula is 
fused below with the tibia. From this form to Hquws is a small 
series of steps, characterised by the still further reduction of all the 
digits except III, by the still further reduction of the already 
rudimentary ulna and fibula, and by the increasing depth of the 
molar teeth, which are of course, in Hquus, hypselodont. 
Another interesting conclusion may seem to follow when we con- 
sider the geographical range of the ancestral Horses. Myracotheriuim 
and Pachynolophus occurred both in the Old and New World. 
From them may have arisen the Horses of both hemispheres. 
After that point there is a division. J/esohippus is American, 
and we get at Hquus in that continent through Desmatippus and 
Protohippus. On the other hand there are no remains known of 
Mesohippus 11 Europe; and unless subsequent researches prove 
the existence of JJesohippus, we have to rely upon forms which 
are placed with Anchitherium and Hipparion. 
It seems that in America the next genus in the direct line 
of equine descent to Mesohippus is Miohippus. It is smaller in 
