Among all the many hoofed mammals of the Uinta and Bridger there was not a single one that had the high-crowned, persistently growing teeth of the grazers; all of them must have had browsing habits and have fed upon such soft vegetable tissue as did not rapidly abrade the teeth. The same statement applies, _à fortiori_, to the stages antecedent to the Bridger and therefore to the entire Eocene and Paleocene. From these facts it may be inferred that the grasses had not yet taken possession of wide areas. Concerning the Bridger fauna, Professor Osborn, who has done so much to elucidate it, says: “On the whole, it is a very imposing, diversified and well-balanced fauna, with an equal distribution of arboreal, cursorial, aquatic, fossorial, carnivorous and herbivorous types.” The lower Eocene is divisible into two stages, in descending order, the Wind River and Wasatch, both extensively exposed in central Wyoming. As would be expected from its stratigraphical position, the Wind River fauna was largely transitional between that of the Bridger above and that of the Wasatch below. Unfortunately, the fossils are far less numerous than those of the Bridger and not so well preserved, and therefore give us a less adequate conception of the life of that time. The archaic, non-progressive orders were strongly represented, but already the progressive groups were in a numerical majority of species; most of these archaic orders may be most advantageously described in connection with the Wasatch. Opossums were almost certainly present, though the available specimens are too fragmentary for assured determination. The †tillodonts, †tæniodonts and insectivores differed little from the Wasatch representatives of these orders, except that the Bridger †tæniodont, _†Stylinodon_, which had rootless, persistently growing teeth, was associated with the Wasatch genus _†Calamodon_. On the other hand, the primitive flesh-eaters, or †creodonts, which were referable to Wasatch families, were less numerous and varied and formed a mixture of Bridger and Wasatch genera. The †Oxyænidæ, the family with cat-like teeth and head, had both the smaller Wasatch genus _†Oxyæna_ and the very large Bridge _†Patriofelis_. Of the blunt-toothed †Mesonychidæ, one very large animal (_†Pachyæna_) survived from the Wasatch. The small forms of the family †Hyænodontidæ were common, and there were numerous species of the progressive family †Miacidæ.