192 The Palaeontological Record. I. Animals 
mals was then so small, that Cuvier’s types were forced into various 
incongruous positions, to serve as ancestors for unrelated series. 
The American family of the Titanotheres may also be distantly 
related to the horses, but passed through an entirely different course 
of development. From the lower Eocene to the lower sub-stage of 
the middle Oligocene the series is complete, beginning with small and 
rather lightly built animals. Gradually the stature and massiveness 
increase, a transverse pair of nasal horns make their appearance and, 
as these increase in size, the canine tusks and incisors diminish 
correspondingly. Already in the oldest known genus the number 
of digits had been reduced to four in the fore-foot and three in the 
hind, but there the reduction stops, for the increasing body-weight 
made necessary the development of broad and heavy feet. The final 
members of the series comprise only large, almost elephantine animals, 
with immensely developed and very various nasal horns, huge and 
massive heads, and altogether a grotesque appearance. The growth 
of the brain did not at all keep pace with the increase of the head 
and body, and the ludicrously small brain may well have been one of 
the factors which determined the startlingly sudden disappearance 
and extinction of the group. 
Less completely known, but of unusual interest, is the genealogy 
of the rhinoceros family, which probably, though not certainly, was 
likewise of American origin. The group in North America at least, 
comprised three divisions, or sub-families, of very different pro- 
portions, appearance and habits, representing three divergent lines 
from the same stem. Though the relationship between the three 
lines seems hardly open to question, yet the form ancestral to all 
of them has not yet been identified. This is because of our still very 
incomplete knowledge of several perissodactyl genera of the Hocene, 
any one of which may eventually prove to be the ancestor sought for. 
The first sub-family is the entirely extinct group of Hyracodonts, 
which may be traced in successive modifications through the upper 
Kocene, lower and middle Oligocene, then disappearing altogether. 
As yet, the hyracodonts have been found only in North America, and 
the last genus of the series, Hyracodon, was a cursorial animal. 
Very briefly stated, the modifications consist in a gradual increase 
in size, with greater slenderness of proportions, accompanied by 
elongation of the neck, limbs, and feet, which become tridactyl and 
very narrow. The grinding teeth have assumed the rhinoceros-like 
pattern and the premolars resemble the molars in form; on the 
other hand, the front teeth, incisors and canines, have become very 
small and are useless as weapons. As the animal had no horns, it 
was quite defenceless and must have found its safety in its swift 
running, for Hyracodon displays many superficial resemblances to 
