362 O. C. Marsh— Vertebrate Life in America. 
represented, and with it is found a nearly allied form, Paleo- 
syops. In the upper Kocene, both have left the field, and the 
genus Diplacodon, a very near relative, holds the supremacy. 
The line seems clear through these three genera, but on 
crossing the break into the Miocene, we have, apparently as 
next of kin, the huge Brontotheride. These strange beasts show 
in their dentition and some other characters the same transition 
steps beyond Diplacodon, which that genus had made beyond 
aleosyops. The Brontotheride were nearly as large as the 
Elephant, but had much shorter limbs. The skull was elon- 
gated, and had a transverse pair of large horn-cores on the 
maxillaries, in front of the orbits, like the middle pair in 
Dinoceras. There were four toes in front, and three behind, 
and the feet were similar to those of the Rhinoceros. There 
are four genera in this group, Brontotherium ; Diconodon ; Meno- 
dus (Titanotherium) ; and Megacerops, which have been found 
only in the lowest Miocene, east of the Rocky Mountains. 
In the higher Miocene beds of Oregon, an allied genus, 
Chalicotherium, makes its appearance, It is one stage further 
on in the transition, and perhaps a descendant of the Bronio- 
theride ; but here,.so far as now known, the line disappears. 
It is a suggestive fact, that this genus has now been found in 
Western America, China, India, Greece, Germany and France, 
indicating thus, as I believe, the path by which many of our 
ancient mammals helped to people the so-called Old World. 
rtiodactyles, or even-toed Ungulates, are the most 
abundant of the larger mammals now living; and the group 
dates back at least to the lowest Eocene. Of the two well 
marked divisions of this order, the Bunodonts and the Seleno- 
donts, as happily defined by Kowalevsky, the former is the 
older type, which must have separated from the Perissodactyle 
line after the latter had become differentiated from the prim- 
itive Ungulate. In the Coryphodon Beds of New Mexico, 
occurs the oldest Artiodactyle yet found, but it is at present 
known only from fragmentary specimens. These remains are 
clearly Suilline in character, and belong to the genus Hohyus. 
In the beds above, and possibly even in the same horizon, 
the genus Helohyus is not uncommon, and several species are 
e molar teeth of this genus are very similar to 
those of the Kocene Hyracotherium, of Europe, which is sup- 
osed to be a Perissodactyle, while Helohyus certainly is not, 
ut apparently a true lineal ancestor of the existing pigs. 
In every vigorous primitive type which was destined to survive 
many geological changes, there seems to have been a tendency 
to throw off lateral branches, which became highly specialized 
and soon died out, because they are unable to adapt themselves 
to new conditions. The narrow path of the persistent Suilline 
