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 _Tragulidae_, or so-called
"Musk-deer," of the present day.

The _Tragulidae_ 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
_Tragulidae_. If _Dichobune_ has a four-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 _Hyopotamus_ and _Chaeropotamus_; but
_Hyopotamus_ appears to have had only two toes.

Again, all the great groups of the Ruminants, the _Bovidae,
Antilopidae, Camelopardalidae_, and _Cervidae_, are represented in
the Miocene epoch, and so are the Camels. The Upper Eocene
_Anoplotherium_, which is intercalary between the Pigs and the
_Tragulidae_, has only two or, at most, three toes. Among the scanty
mammals