The fauna of the Snake Creek formation in western Nebraska and that of the presumably somewhat later beds of northwestern Nevada, which are referable to the lower Pliocene, may be considered together. The rodents, which are not very fully represented, were quite modern in character and belonged mostly to extinct species of modern genera, such as hares, pocket-gophers, beavers, forerunners of the †Giant Beaver, marmots, sewellels, etc. A remnant of a more ancient world, especially characteristic of the Miocene, is found in the remarkable burrowers, the horned †mylagaulids which have been extinct since the lower Pliocene. Carnivora were abundant, and members of all the families which inhabit North America to-day have been obtained; wolves, “†bear-dogs,” “†hyena-dogs” and forms like the Dhole of India were common. The terms “†bear-dogs” and “†hyena-dogs” are not to be understood as implying any relationships of these animals to bears or hyenas, but merely a certain superficial resemblance; these were very large members of the dog family (Canidæ), now extinct. Mustelines, large and small, are found, and possibly some bears had already made their way from the Old World, but this is still uncertain. †Sabre-tooth tigers and true cats, some as large as lions and one species fairly gigantic, were likewise characteristic of the time. There was a great wealth of horses, though the modern genus _Equus_ was not among them; all the genera are now extinct and all were three-toed. Several distinct phyla were represented, some progressive and advancing toward the modern forms, others conservative and stationary. Browsing horses with low-crowned teeth, grazing horses with prismatic, cement-covered teeth, heavier and lighter, larger and smaller, must have covered the plains and thronged the woods. Ancestral tapirs were present, though far less common. A family which seems to be utterly exotic to North America, that of the rhinoceroses, was present, and of these there were three or four series, mostly without horns, or with a very small horn on the tip of the snout. The extremely aberrant perissodactyls (†Ancylopoda), in which the hoofs were converted into great claws, perhaps persisted, but the evidence is not conclusive.