92 



NORTH AMERICAN FAUNA 74 



However, even the thinnest annual layers are thicker than those on the teeth of 

 most other mammals, and the total accumulation on the teeth of old walruses is, 

 thus, exceptionally great. The maximum thickness of accumulated increments on 

 the second upper premolar in two old males, 38 and 40 years old, was 10 and 

 11 mm, respectively. In those teeth, more than 90% of the total volume of 

 calcified tissue was cementum. 



Shape, Orientation, and Occlusion of Cheek Teeth 



The dentition of the walrus, apart from the tusks, is made up of simple, peg- 

 like (haplodont) teeth, of which only 16 



are consistently present and functional. When fully developed, these are mostly 

 2.5 to 5.5 cm long and 1.0 to 2.5 cm in diameter. The most deeply rooted of these 

 usually are P and C^; P2 usually is the largest in diameter. These teeth range in 

 cross-sectional outline from nearly round to slightly elliptical, the former being 

 more typical of the upper and the latter more typical of the lower teeth. They 

 taper to a blunt point at the proximal end (root apex), and the distal end (crown) 

 ordinarily becomes obliquely beveled (upper teeth) or somewhat chisel-shaped 

 (lower teeth) (Fig. 53). 



The upper cheek teeth usually are somewhat longer than the lower, their 

 combined lengths making up more than 50% of the total combined lengths of all 

 of the teeth. This proportion increases with advancing age (Fig. 59), for all of the 

 lower teeth tend to decrease in relative length, whereas some of the upper teeth 

 increase (P2, P3), decrease (P), or remain stationary (Fig. 60). Evidently, P has 

 the highest rate of decrement of any of the teeth; P2 has the lowest rate and 

 becomes the longest tooth in the mouth. The rapid diminution of P, usually the 

 most anterior tooth in the mouth, suggests that it has the greatest contact with 

 abrasive materials. 



The orientation and occlusion of the teeth were studied in some detail by Cobb 

 (1933), mainly in skulls of adult males. He observed that the teeth are not set 



60 



55 



50 



10 



Age 



20 

 (years) 



30 



Fig. 59. Combined lengths of the 

 upper cheek teeth (UCL), ex- 

 pressed as percentage of the total 

 combined lengths (TCL) of all of 

 the cheek teeth, in relation to age. 

 Each point represents one speci- 

 men. 



