actyles, or odd-toed ungulates.

"In the next higher division of the Eocene another genus,
Orohippus, makes its appearance, replacing Eohippus, and showing
a greater, though still distant, resemblance to the equine type.
The rudimentary first digit of the forefoot has disappeared, and
the last premolar has gone over to the molar series. Orohippus
was but little larger than Eohippus, and in most other respects
very similar. Several species have been found, but none occur
later than the Upper Eocene.

"Near the base of the Miocene, we find a third closely allied
genus, Mesohippus, which is about as large as a sheep, and one
stage nearer the horse. There are only three toes and a
rudimentary splint on the forefeet, and three toes behind. Two
of the premolar teeth are quite like the molars. The ulna is no
longer distinct or the fibula entire, and other characters show
clearly that the transition is advancing.

"In the Upper Miocene Mesohippus is not found, but in its place
a fourth form, Miohippus, continues the line. This genus is near
the Anchitherium of Europe, but presents several important
differences. The three toes in each foot are more nearly of a
size, and a rudiment of the fifth metacarpal bone is retained.
All the known species of this genus are larger than those of
Mesohippus, and none of them pass above the Miocene formation.

"The genus Protohippus of the Lower Pliocene is yet more equine,
and some of its species equalled the ass in size. There are
still three toes on each foot, but only the middle one,
corresponding to the single toe of the horse, comes to the
ground. This genus resembles most nearly the Hipparion of
Europe.

"In the Pliocene we have the last stage of the series before
reaching the horse, in the genus Pliohippus, which has lost the
small hooflets, and in other respects is very equine. Only in
the Upper Pliocene does the true Equus appear and complete the
genealogy of the horse, which in the Post-Tertiary roamed over
the whole of North and South America, and soon after became
extinct. This occurred long before the discovery of the
continent by Europeans, and no satisfactory reason for the
extinction has yet been given. Besides the characters I have
mentioned, there are many others in the skeleton, skull, teeth,
and brain of the forty