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 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 of the Lower Eocene formation we have the perissodactyle Ungulata represented by Coryphodon, Hyra-cotherium, and Pliolophus.  Suppose for a moment, for the sake of following out the argument, that Pliolophus represents the primary stock of the Perissodactyles, and Dichobune that of the Artiodactyles (though I am far from saying