SKELETON AND SKULL. 731 



the deep longitudinal grooves on each side. Not unfrequently, 

 however, the fourth upper molar is either distinctly double- 

 rooted, or incipiently so, the two fangs being connate and more 

 or less completely united. It sometimes happens that the 4th 

 upper molar is double-rooted on one side and single-rooted on 

 the other. In old age the bones of the skull become greatly 



FIG. 56. Cystophora cristata. No. 6576, Nat. Mus.; | nat. size. 



thickened, the surface assumes a more or less rugose charac- 

 ter, with incipient sagittal and well-developed occipital crests. 

 The sutures, even in old age, remain open nearly throughout 

 the skull. 



The usual sexual variations are observable in the smaller 

 size, weaker structure, and smoother surface of the bones in the 

 females. An adult female skull gives a length of 220 mm. 

 against a length of 275 mm. in an old male. Detailed measure- 

 ments are given in the subjoined table. 



