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.