24 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 6 1 



effect on the nasal boss is chiefly that of expansion with very little 

 increase in length. This is well shown by the relationship of the width 

 to the length in the various ages. In the nursing calf the length 

 greatly exceeds the width ; in the immature, with milk dentition still 

 in use, these two dimensions are about equal; while in the old the 

 width greatly exceeds the length. There is also a general increase 

 in the size and massiveness of the paroccipital and post-glenoid pro- 

 cesses as age advances. In adults these processes show considerable 

 elongation and attenuation at their tips together with some increase 

 in rugosity. The post-orbital breadth or constriction of the skull 

 attains its size early and remains fixed, thus becoming in age propor- 

 tionally much less. The crests and processes are the parts which 

 finally give the skull its greatest size, and these are the portions which 

 change most with age. The greatly developed lambdoidal crests 

 show a gradual increase in breadth across the occipitals as age 

 advances. 



An invariable feature of this species is the single infra-orbital 

 foramina. No exceptions in this character have been detected in any 

 skulls. In Diceros we find every degree of variation in number from 

 one to three openings between individual skulls and between the two 

 sides of the same skull. The same variation exists to a less degree 

 in Hippopotamus and Elephas. 



The skull also shows a very considerable amount of sexual varia- 

 tion, which renders it comparatively simple to determine the sex of 

 an individual by attention to such characters. The nasal expansion is 

 the most marked character of this sort. In the adult male this expan- 

 sion is constantly 20 mm. or more greater than in the female skulls, 

 while the length of the nasal projection beyond the maxillae is cor- 

 respondingly greater and ranges from 15 mm. to 25 mm. greater. 

 The greater width of the nasal expansion in .the male is directly in 

 accord with the much more massive front horn of the male. The 

 general massiveness of the skull is considerably greater in the male 

 where it is especially marked by the greater length. The male skulls 

 exceed female skulls from 40 mm. to 50 mm. in length, and also 

 average considerably greater in breadth. Accompanying this in- 

 creased size, is greater bulk to the bones forming the zygomatic arch, 

 the depth of the zygomatic processes of the squamosal being much 

 greater in the male skulls. The coronoid process of the mandible- is 

 somewhat shorter in female skulls, and the mandible itself shows con- 

 siderably greater length in the male. The teeth, however, exhibit no 

 differences in size or shape in the .two sexes. 



