During Blancan (late Pliocene) times, rabbits were an important and varied part of prairie faunas. Screen-washing of fossiliferous sediments often yields abundant remains of rabbits, commonly of several different genera, and these remains present problems in identification. By the Pliocene, rabbits had evolved completely hypsodont, evergrowing teeth. The rootless teeth, in the adult, were of uniform diameter throughout their lengths and readily slipped from their alveoli in the decomposing carcasses of dead animals. Isolated teeth and toothless jaw fragments are the fossils usually recovered. Fortunately the anterior lower premolar tooth (P3) alone is usually sufficient for identification to genus. Criteria for separation of the lower P3 of Nekrolagus and Sylvilagus-Lepus from Pi4atilepus were given by Hibbard (1963), and other papers have listed means of separating lower P3's of other genera. The publications involved, however, are devoted to taxonomic and evolutionary discussions, and the characters critical for identification of genera are rarely emphasized or are buried in detailed descriptions. The following is intended to permit the swift and simple generic identification of Blancan rabbit lower P3's in almost all instances. Criteria involve only the lower P3. The relationship of the genus Aluralagus suggested here is original. Through most of the Miocene and early Pliocene (Hemphillian) time, the common rabbits of North America had rather heart-shaped P3's. This tooth form is typified by Hypolagus vetus (Kellogg). The enamel was of rather uniform thickness and there were simple, straight, cementum-filled anteroexternal and posteroexternal valleys (folds), the former the shorter and the latter not reaching half of the distance across the tooth. In late Hemphillian time some environmental change, perhaps in food plants, seems to have placed increased stress on the P3. Blancan species of Hypolagus evolved longer external valleys, sometimes directed more forward or backward rather