From the White River, or lower Oligocene, many well-preserved rhinoceroses, including complete skeletons, have been gathered in the various collections and display very interesting differences in the three substages of the White River beds. In the uppermost substage is found the apparent beginning of the †dicerathere phylum, though it may be traced back to the middle substage; the nasal bones had become much thickened so as to serve as a support for the horns, and these are indicated by a small, but very rough, area on the outer side of each nasal. Comparing this White River species with those of the upper Oligocene and lower Miocene, two differences may be observed: in the later species the horn-supports were well defined bony knobs or prominences, and these knobs were close to the anterior ends of the nasals; while in the White River animal the places for the attachment of the horns were mere roughened areas, and these were well behind the tips of the nasals. This is not an infrequent sort of change, that horns should shift their position forward or that the portion of the nasals in front of the horns should be shortened. Parallel changes occurred among the †titanotheres. In the middle White River all the rhinoceroses were hornless, but the same two phyla may be distinguished; the actual starting point of the †diceratheres had no indication of the nasal horns, but may be identified as such by their close resemblance in other respects to the species of the upper substage in which the incipient horns appeared. Much commoner were the members of the typical hornless line (see Fig. 135, p. 256), which, though true and unmistakable rhinoceroses, were yet far removed in many details of structure from the progressive genera of the middle and upper Miocene. There are several species in this phylum, which constitute a series of diminishing size almost in proportion to their increasing antiquity. The dentition was already thoroughly and characteristically rhinoceros-like, but a more primitive feature was the presence of a second upper incisor, a small tooth placed behind the trenchant one, making the incisor formula 2/2; the third incisor and the canines of both jaws were already lost. The assumption of the molar-pattern by the premolars varied much in degree of completeness in the different species; the upper molars, while having all the essentials of the rhinocerotic plan of structure, had a much less complex appearance than in the Recent genera, because of the absence of the accessory spurs; and all the grinding teeth were very low-crowned, in strong contrast to the high-crowned (yet not properly hypsodont) teeth of the middle Miocene and subsequent genera. [Illustration: FIG. 174.—Skull of †hornless rhinoceros (_†Cænopus tridactylus_); middle White River stage. (After Osborn.)] [Illustration: FIG. 175.—Second upper molar, left side, of _†Cænopus_, showing the masticating surface.]