t have had a five-toed "atavus" at some earlier period.

No such five-toed "atavus," however, has yet made its appearance among
the few middle and older Eocene _Mammalia_ which are known.

Another series of closely affiliated forms, though the evidence they
afford is perhaps less complete than that of the Equine series, is
presented to us by the _Dichobune_ of the Eocene epoch, the
_Cainotherium_ of the Miocene, and the _Tragulidoe_, or so-called "Musk-
deer," of the present day.

The _Tragulidoe_; have no incisors in the upper jaw, and only six
grinding-teeth on each side of each jaw; while the canine is moved up to
the outer incisor, and there is a diastema in the lower jaw. There are
four complete toes on the hind foot, but the middle metatarsals usually
become, sooner or later, ankylosed into a cannon bone. The navicular and
the cuboid unite, and the distal end of the fibula is ankylosed with the
tibia.

In _Cainotherium_ and _Dichobune_ the upper incisors are fully developed.
There are seven grinders; the teeth form a continuous series without a
diastema. The metatarsals, the navicular and cuboid, and the distal end
of the fibula, remain free. In the _Cainotherium_, also, the second
metacarpal is developed, but is much shorter than the third, while the
fifth is absent or rudimentary. In this respect it resembles
_Anoplotherium secundarium_. This circumstance, and the peculiar pattern
of the upper molars in _Cainotherium_, lead me to hesitate in considering
it as the actual ancestor of the modern _Tragulidoe_. If _Dichobune_ has
a fore-toed fore foot (though I am inclined to suspect that it resembles
_Cainotherium_), it will be a better representative of the oldest forms
of the Traguline series; but _Dichobune_ occurs in the Middle Eocene, and
is, in fact, the oldest known artiodactyle mammal. Where, then, must we
look for its five-toed ancestor?

If we follow down other lines of recent and tertiary _Ungulata_, the same
question presents itself. The Pigs are traceable back through the Miocene
epoch to the Upper Eocene, where they appear in the two well-marked forms
of _Hyopopotamus_ and _Choeropotamus_; but _Hyopotamus_ appears to have
had only two toes.

Again, all the great groups of the Ruminants, the _Bovidoe, Antilopidoe,
Camelopardalidoe_, and _Cervidoe_, are represented in the Miocene epoch,
and so are the Camels. The Upper Eocene _Anoplotherium_, which is
intercalary between the Pigs and the _Tragulido