The †proterotheres, on the other hand, were almost the only Santa Cruz ungulates which had nothing _outré_ or grotesque about them to the eye of one habituated to the faunas of the northern hemisphere. They were small, graceful animals, very like the Miocene horses of the north in their proportions, though having much shorter necks and shorter, heavier heads. In some genera of this family (_e.g._ _†Diadiaphorus_, _†Proterotherium_) the feet were three-toed and most surprisingly horse-like in shape, but one genus (_†Thoatherium_) was absolutely single-toed, more completely monodactyl than any horse. The horse-likenesses ran all through the skeleton and are so numerous and so striking that several writers have not hesitated to incorporate the †Litopterna with the Perissodactyla, but this I believe to be an error. If the †proterotheres were not perissodactyls, as I am convinced they were not, they afford one of the most remarkable examples of convergent evolution among mammals yet made known. 3. _Oligocene_ _North America._—The John Day formation of eastern Oregon represents the upper Oligocene and has yielded a very extensive series of mammals, though with some obvious gaps that remain to be filled by future work. The land-connection with the Old World which had existed in the lower Oligocene and was restored in the lower, or at latest in the middle, Miocene, was interrupted in John Day times, and so the mammals assumed a purely indigenous character. No opossums or other marsupials have been found, and nothing is known of the Insectivora. Of the Carnivora, there were but three families, and one of these, the mustelines, was represented but scantily by a few small species. Cats of the †sabre-tooth subfamily were common and one species was quite large, almost equalling the Jaguar in length; but most of the species were small, much smaller than the Pleistocene members of the group. True cats are not definitely known to have been present, but there were two genera (_†Nimravus_ and _†Archælurus_) which have been called the “false †sabre-tooths,” which may prove to be referable to that series. The dogs, on the other hand, were remarkably numerous and diversified, more so than ever before or since; none of them was very large, the largest but little exceeding the Timber Wolf in size, and some were extremely small; but the number of distinct genera and species and the differences among them are quite remarkable. Both long and short-faced forms and early stages of the “†bear-dogs,” and “†hyena-dogs,” and ancestral forms of the wolves and dholes may be distinguished, a truly wonderful assemblage. The rodents also were numerous and varied, including ancient and extinct genera of the beavers, squirrels, mice, pocket-gophers and hares and the earliest distinguishable ancestors of the sewellels (Aplodontiidæ).