334 LAND MAMMALS IN THE WESTERN HEMISPHERE 



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 fdicerathere 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 dif- 

 ferences 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 ftitan- 

 otheres. 



In the middle White River all the rhinoceroses were horn- 

 less, but the same two phyla may be ciistinguished ; the actual 

 starting point of the fdiceratheres 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 mem- 

 bers of the typical hornless line (see Fig. 135, p. 256), which, 

 though true and unmistakable rhinoceroses, were yet far re- 

 moved 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 al- 

 most 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. 



