86 



NORTH AMERICAN FAUNA 74 



months old. Data from the captives were provided by C. A. Hill (personal 

 communication) of the San Diego Zoological Society. The order in which the 

 teeth erupted through the gingiva and the ages (months) at which they first 

 emerged were Ci (2-i), Ci (2-6), P3 (3-8), I3 (4-12), P2 (8-12), P (10-12), P2 

 (11-13), P3 (12-15), P4 andPl (13-15), P (14-15), P4 (36-48). More than half of 

 the teeth made their appearance in the first year after birth. By the end of the 

 second year, the entire anatomical crown of each secondary tooth, except P4, 

 usually was gingivally erupted and occluded with its antagonist. 



The percentage frequency of occurrence of gingivally erupted secondary teeth 

 and primary molars in 136 specimens is shown in Table 11. 



General Structure of Secondary Teeth 



Each secondary tooth in the walrus calf at birth consists of a delicate, hollow 

 cone of primary dentin capped with enamel and containing the dental pulp (Fig. 

 53a). The first thin layer of cementum is deposited on the exterior of the 

 anatomical root, proximal to the enamel, in the second or third month after 

 birth, well before most of the teeth have begun to erupt but about the time when 

 gingival eruption of the canines begins. Development of the enamel cap on the 

 anatomical crown of each tooth is completed at or soon after birth, before 

 gingival eruption of the tooth. This cap, about 0.25 mm thick, is very fragile and 

 is deleted by attrition and abrasion within a few years after eruption of the tooth. 

 Usually, littie or no enamel remains on any of the teeth, 6 or 7 years after 

 gingival eruption. 



Table 11. Percent frequency of occurrence of gingivally erupted secondary 

 teeth and primary molars in relation to age.^ 



Age in years ^PP^^ 



from birth No. II 12 13 CI PI P2 P3 P4 ml 



0 



27 



0 



0 



0 



0 



0 



0 



0 



0 



0 



1 to 1.6 



11 



9 



36 



64 



100 



54 



73 



36 



0 



9 



2 to 2.6 



10 



0 



30 



100 



100 



100 



80 



100 



0 



10 



3 to 6 



18 



0 



39 



100 



100 



100 



89 



89 



28 



17 



7 to 10 



19 



0 



42 



100 



100 



100 



100 



95 



47 



11 



11 to 15 



27 



0 



26 



100 



100 



96 



100 



100 



15 



0 



16 to 30 



24 



0 



4 



96 



100 



100 



100 



100 



12 



4 















Lower 











0 



27 



0 



0 



0 



0 





0 



0 



0 



0 



1 to 1.6 



11 



0 



0 



36 



91 





82 



73 



45 



0 



2 to 2.6 



10 



0 



0 



0 



100 





100 



100 



70 



10 



3 to 6 



16 



0 



0 



12 



100 





100 



100 



88 



19 



7 to 10 



17 



0 



0 



6 



100 





100 



100 



100 



0 



11 to 15 



26 



0 



0 



8 



96 





100 



100 



100 



0 



16 to 30 



22 



0 



0 



0 



100 





100 



100 



100 



9 



^Unilateral, as well as bilateral occurrence, was scored as "present." 



