aeotheres, confined to the Eocene and Oligocene of
Europe, dying out without descendants. In the earlier attempts to work
out the history of the horses, as in the famous essay of Kowalevsky
("Sur l'Anchitherium aurelianense Cuv. et sur l'histoire paleontologique
des Chevaux", "Mem. de l'Acad. Imp. des Sc. de St Petersbourg", XX. no.
5, 1873.), the Palaeotheres were placed in the direct line, because
the number of adequately known Eocene mammals 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 will 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 proportions, 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 Eocene, any one of which may
eventually prove to be the ancestor sought for.

The first sub-family is the entirely exti