ON THE EXTINCT ANIMALS OF NORTH AMERICA. 
281 
and Lophiodont forms had nearly, if not quite, died out, but 
the more horse-like Mesohippus , Miohippus , and Anchither- 
ium were abundant, and appear to continue the line from the 
Eocene Orohippus to the true horses of the Pliocene period. 
Ehinocerotic forms now became ascendent, being represented 
by Diceratherium (Marsh), differing from all existing animals 
of the group, by having a pair of horns placed side by side on 
the nasal bones; and a very interesting genus, Hyracodon 
(Leidy), an animal with molar teeth and many other of the 
characters of rhinoceros, but having no nasal horn, and having 
a complete set of incisor and canine teeth, as in all the older 
Perissodactyles, which are lost in the more modern rhinoceros. 
It is therefore quite a connecting link between the Palseo- 
theroid animals of the Eocene, and the true rhinoceros of the 
Pliocene, and occupies exactly the right geological horizon that 
such a form ought to do if the one has been genetically derived 
from the other. 
Hyracodon , therefore, has a high place of interest among 
many of similar nature which have been revealed by our newly- 
acquired knowledge of the ancient American fauna. If, how- 
ever, as is stated, the fifth digit of the fore-foot is only rudi- 
mentary, it could scarcely have been, as remarked by Marsh, 
on the direct line of descent from the four-toed Eocene to the 
equally four-toed Miocene rhinoceros, though certainly in such 
a case we know not what ought to be allowed for reversion. 
The same period (generally speaking) also produced several 
species of a more perfect rhinoceros, still hornless, however, 
and resembling the contemporaneous European Aceratherium. 
But the most remarkable of the Miocene Perissodactyles, 
and in some respects the most remarkable of all the animals 
which the recent explorations have brought to light, are a 
number of species of gigantic size, to the first known of which 
Leidy gave the name of Titanotherium , and of which other 
forms have been named by Marsh Brontotherium , and by Cope 
Symborodon . 
They must, by their great size and strength, grotesque ap- 
pearance, and general mode of life, have taken the place in the 
Miocene times of the then extinct Uintatherium of the 
Eocene, and were in their turn replaced by the Mastodons 
and Elephants of later ages. The rhinoceros of the present 
day will serve to give the best general idea of the appearance 
of these creatures, but some of them (for they seem to have 
been numerous both in species and individuals) approached 
nearer to the elephant in size and length of limb. The skull 
(PI. CXXXYIII. figs. 1 and 2) in its general characters was 
quite rhinocerotic ; but the nasal bones supported a pair of 
large, laterally divergent, rugged prominences, apparently for 
