The line of division between the lower Miocene and the uppermost Oligocene is a very obscure and difficult one to draw. Personally, I prefer to begin the Miocene with the widespread formation of the Great Plains, which has been variously named Arikaree, Harrison, Rosebud, etc., but this is a moot point. Concerning the lower part of these beds Osborn says: “They may be either: (1) Upper Oligocene or (2) transitional from Oligocene to Miocene, or (3) of pure Lower Miocene age.” The upper division is referred to the Miocene without question by any one, but for the purposes of this rapid sketch it will be best to treat the two faunas together. This many-named formation, for which the term _Arikaree_ is here employed, as having priority, is found over extensive areas of South Dakota, northern Nebraska and central Wyoming. The fauna was almost entirely a development from that of the North American Oligocene, with very little admixture of foreign elements, so that the land communication with the eastern hemisphere must have been difficult. In this, as in most of the Miocene formations, the smaller mammals are not fairly represented, and it is evident that much remains to be learned with regard to them; this is especially true of the upper division of this stage. [Illustration: FIG. 128.—Most ancient American Antelope (_†Dromomeryx antilopina_), middle Miocene. Restored from specimens in the Carnegie Museum and Princeton University.] The rodents, which were fairly numerous, were directly continuous with those of the upper Oligocene and included forms which were more or less distantly connected with the modern hares, squirrels, beavers, sewellels, pocket-gophers and kangaroo-rats. A few Insectivora of doubtful reference have been found. Among the Carnivora there was also considerable variety: dogs, large and small, were abundant, but all of them were decidedly primitive from the modern standpoint; the cats were represented both by the true felines, which were probably immigrants, and by the †sabre-tooth series. There were several large and powerful mustelines, or members of the weasel family, which were likewise immigrants, one of which resembles in many ways the modern Wolverene (_Gulo_). Very interesting is the beginning of the raccoon family (Procyonidæ) or, at least, what is believed to be such, which arose from a branch of the dogs; this most ancient of the raccoons was _†Phlaocyon_, a small and slender animal.