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p. 181-295, [7] leaves of plates (2 folded) : ill., maps ; 25 cm. ; Includes bibliographical references (p. 290-295). ; "1. The Perissodactyla show a very early and very fundamental dichotomy into two suborders, for which the names Solidungula and Tridactyla could be appropriately revived. The family Hyrachyidae is a very primitive group of the Rhinocerotoidea, and still closely resembles the other Eocene families of the suborder Tridactyla. The family is known from the Eocene (Lost Cabin to Uinta, inclusive) of Wyoming, Utah, Colorado and Montana. It includes four genera, of which Hyrachyus is by far the best known. Hyrachyus has the greatest time range of any American Eocene ungulate genus, and has an unusually long range for a placental mammal. The taxonomy, stratigraphic distribution and probable phylogeny are summarized in figure 1. 2. The genus Hyrachyus apparently includes three contemporary main series (consisting of small, medium-sized and large forms, two of which series have side branches, of which a few are rather aberrant in one or more characters). Each series contains successive grades of organization which are best treated as species. It is significant that when known from sufficiently numerous specimens, the individual measurements of the representatives of each species form normal curves of distribution, with relatively slight, or no, overlapping with other contemporary forms, or with earlier or later forms. Certain of these forms are now sufficiently well documented to deserve rank as index fossils for their respective horizons. H. modestus is an index fossil for the Lower Bridger, H. eximius and H. princeps for the Upper Bridger, and the genus Hyrachyus is very characteristic of the Bridger in general, although not confined to it. 3. Paired horns appear independently in the second and third genera of the family, Colonoceras and Metahyrachyus. Colonoceras is close to H. affinis. Metahyrachyus is a rather aberrant form, particularly as to P[superscript]2, in which the peculiar 'amphiloph' ...