VARIATIONS DEPENDENT UPON AGE, ETC. 45 



The frontals vary greatly in form at their posterior border, 

 especially in respect to their interparietal extension. Tliis por- 

 tion has sometimes a breadth eqnal to that of the nasal bones, 

 and terminates quite squarely ; at other times, it has less than 

 half this breadth, and is rapidly narrowed posteriorly. 



The tusks vary considerably iu length, size, and form, and 

 more especially in direction, in specunens of the same age and 

 sex. They become much larger in old age than in middle life, 

 but are then more or less abraded and broken at the points. 

 The grooving on the sides varies more or less with each indi- 

 vidual, and even in the two tusks of the same animal. The 

 tusks generally widely diverge, but are sometimes nearly par- 

 allel, but apiDear to be very rarely convergent, while in the 

 female they are frequently more or less convergent, and some- 

 times touch at the points, or even overlap. 



In regard to external characters, considerable changes result 

 fi?om age, especially in respect to the size and amount of abra- 

 sion of the tusks, and through the loss of the hair incident to 

 old age, and the shortening of the mystacial bristles. 



The following table of measurements (given in millimeters) 

 shows to some extent the variations that occur in the general 

 size and form of the skull. 



