NO. I THE WHITE RHINOCEROS — HELLER 25 



The individual variation seems to be greatest along the lines of 

 generic differences. One of the most variable characters of this sort 

 is the amount of separation between the post-glenoid and post- 

 tympanic processes. This varies independent of age or sex from 

 2 mm. to io mm., and shows an average variation on the two sides of 

 the individual skulls of 50 per cent. The wide variation in the char- 

 acter of the separation throws much doubt upon its generic value. 

 The vomer in one specimen, number 164589, shows a marked depar- 

 ture in thickness, or width, over the normal, thin knife-like shape of 

 the bone. There is also shown a small amount of individual variation 

 in the extent of the lambdoidal prolongation of the occipitals and in the 

 amount of union between the extremities of the premaxillse, but the 

 great bulk of the variation in these bones is due to age. The width 

 of the parietal flat area shows great individual variation with a rela- 

 tively slight increase due to age. We find the same individual varia- 

 tion occurring in the bush-pig, Potamochcerus, where this feature of 

 the skull is much better marked. 



The mandible shows much individual variation in the depth of the 

 ramus at the angle, but this feature, however, averages greatest in the 

 males. Length of symphysis also shows much individual variation. 



There are four milk molars on each side in both the upper and lower 

 series. The first milk molar, both in the upper and the lower jaw, 

 is not replaced by a permanent tooth, but is gradually crowded for- 

 ward and lost soon after the permanent series is well established. 

 The loss of this tooth occurs some time before the shedding of the last 

 milk molar, but a pit in front of the second premolar, representing the 

 alveolus of the tooth, remains a long time afterward. 



The milk molars are succeeded by the permanent premolars from 

 before backwards with the exception of the first, which is never 

 replaced. The first milk molar to be replaced by a premolar is the 

 second. Soon afterward the third is replaced, but the fourth persists 

 much longer and is not usually replaced until the last molar is begin- 

 ning to appear. Contrary to the usual rule in ungulates, the milk 

 molars show slightly less size and complexity than the permanent 

 series which replaces them. The anterior teeth of the milk series are 

 approximately only half the size of the permanent ones. At the time 

 of their eruption their crowns are marked by sharp unworn ridges of 

 enamel with two deep valleys extending into them from the inner bor- 

 der. Soon by the union of the two accessory crests, the crests from 

 the ectoloph and the crotchet from the metaloph, an island of enamel, 

 the medifossette is cut off from the anterior valley and isolated in the 



