128 



NORTH AMERICAN FAUNA 74 



Fig. 87. Frontal abrasion and fracture of tusks of female Pacific walruses in old age: 

 left, tusks of a 25-year-old female in which the distal third of the crown has been weak- 

 ened by frontal abrasion; right, old females from which the distal part of the weakened 

 tusks has been lost due to bilateral fracture. (Photos by F. H. Fay) 



The posteromedial surface is least affected and often remains nearly intact all the 

 way to the tip. (Fig. 8Sb). The posterior surface sustains about twice as much 

 tissue loss as the medial surface, and the lateral surface about six times as much. 

 The principal vector of abrasive force affecting the tusks is at an angle of about 

 25° from the sagittal plane of the head, rather than parallel to that plane (Fig. 

 89b). This vector is exactly perpendicular to the frontal plane of each side of the 

 snout, immediately above the tusks (Fig. 89a). 



The anterolateral surface of each tusk is densely pitted by the most direct 

 impact of abrasive particles (Fig. 90, left). The principal direction of movement 

 of those particles along that surface is distad, as indicated by the striations made 

 by the particles. On the lateral and medial surfaces, the striae indicate 

 posterodistad movement of the abrasives. The striae there mostly are oriented at 

 10 to 50° from the long axis of the shaft; the predominant angle is about 35° 



