116 



NORTH AMERICAN FAUNA 74 



2.0r- 



1.0 



2. Or 



I .5 



1.0 



J \ L 



J \ \ 3_L 



o o 



^^''05> ooo o^ocP 



■~~-0 



0 10 20 30 40 50 60 70 

 Length Clinical Crown (cm) 



80 



Fig. 80. Change in cross-sectional shape of the tusks, in relation to length of the clin- 

 ical crown in Pacific walrusas. Shape of the cross -sectional ellipse is indicated by the ratio 

 of diameters on the long (A-P) to the short (M-L) axis, measured at the gingival margin. 

 Each point represents one male (dot) and female (circle). Curves were fitted visually. 



in a continual increase in length as well as in mass with increasing age. The 

 principal tissue added is dentin, which is deposited on the walls of the pulp 

 chamber in dense layers, and in the central core of the tusk in the form of coa- 

 lescing globules, as noted earlier. Thin layers of cementum are deposited on the 

 external surface of the clinical root, adding further to the mass but not to the 

 length of the tusk. Ideally, the growth of the tusks should be described in terms of 

 volume or mass. However, for reasons of expediency, I measured only the 

 growth in length. 



The length of each tusk is a segment of an arc, rather than a straight line. For 

 that reason, I measured each specimen along the arc of the anterior surface of the 

 tusk, from the gingival margin to the distal tip. This is the length of the clinical 

 crown or "exposed length." From many specimens, I was able to remove the tusk 

 from its socket and measure the length of the clinical root as well, also along the 

 anterior surface from the gingival margin to the apical end. The sum of those 

 two measurements is the total length of the tusk. The total length of the extant 

 tusk, plus the length of material abraded or fractured from its distal end is the 

 sum of linear increments. This is the total length that would have been present 

 had there been no decrements. The linear amount of decrement was estimated 

 by direct, visual comparison of abraded or broken tusks with others that were of 

 comparable form but from which little or no tissue had been removed. 



Each tusk measurement was correlated with the age of the animal. The results 

 were as follows: 



Length of clinical crown. — The tusks first penetrate the gingiva in the summer 

 or autumn after birth; in the next 15 years, the clinical crown increases in length 



