ECOLOGY AND BIOLOGY OF THE PAGIFIC WALRUS 



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oblique to the long axis of the tooth. The striations made by abrasive particles on 

 the surface of the labial facet rarely are as deep as those on the lingual facet; 

 furthermore, they differ also in that they are inscribed vertically, parallel to the 

 vector of occlusive force (Fig. 66c). The abrasion of this facet apparently takes 

 place as a consequence of contact with the gingiva lateral to the upper cheek 

 teeth and with that on the medial side of the base of the tusk (Figs. 61, 67). The 

 labial facet occurs only in the presence of such contact and always is largest 

 where that contact is greatest (on P2). Very fine, microlithic particles, caught 

 between the tooth and the gingiva, as the jaw is moved up and down, probably 

 are responsible for all of the vertical striae. Tiny fragments of clam shells also 

 could be involved, for the dentin and cementum are soft enough to be scored by 

 such fragments. If the abrasion on this facet were entirely the result of clam shells 

 being crushed in this area, a complementary facet should be produced on the 

 medial -distal surface of the opposing tooth, when the occlusion is like that on the 

 right side of Fig. 6L However, no such facet is produced; the medial surface of 

 those distal flanges on Pl and P2, although nearly parallel to the lingual facets of 

 P2 and P3, is highly polished and bears fine horizontal striae, perpendicular to 

 those on the labial facets. Thus, the abrasion on the distal flanges is the same as 

 that on the lingual facet (Fig. 66b). 



The fourth type of abraded surface is the incisive facet. The abrasive process 

 that produces this facet affects only the crowns of the vestigial incisors and, occa- 

 sionally, the anteromedial surface of P and Gj. The incisive facet is pitted, 

 somewhat like the occlusal facet, from which it differs, however, in that (1) there 

 is no occlusion with an opposing tooth, (2) the facet is not delimited by a 

 peripheral ridge, (3) the edges of the pits are rounded, and (4) the surface 

 between the pits is polished (Fig. 66d). 



The incisive facet on the vestigial incisors occupies the entire clinical crown, 

 which is worn down level with the gingiva (Fig. 66, inset). On those incisors, its 

 surface is not only pitted but is deeply incised as well, the coarse striae being 

 mainly parallel with the long axis of the oral cavity and tongue, as if made by 

 coarse abrasives moving into and out of the mouth. Where this facet occurs on P 

 and on Gj, it may occupy up to 10% of the anteromedial surface of the crown. 

 There, as on the vestigial incisors, it also has many coarse striae, but these range 

 in orientation from oblique (60 to 70°) to parallel with the vertical vector of 

 percussive force. On P and Gj, the posteromedial border of the incisive facet 

 often blends with the anteromedial border of the lingual facet, indicating that 

 the rounding and polishing of its pits and striae are the result of lingual abrasion. 



If any of these four types of abraded facets were the result of crushing and 

 mastication of molluscan shells, as has been assumed previously (Gobb 1933), 

 they should be absent from the teeth of walruses reared in captivity on a diet of 

 fishes and shell-free clam meats. With this in mind, I examined the teeth of eight 

 such specimens, ranging in age from 2.4 to 15.3 years and found the following: 



• In all animals, the occlusal facets were similar to those on the teeth of free- 

 living walruses. 



• In the five oldest animals, the lingual facets were nearly the same as those of 

 free-living walruses, except that they usually were not as highly polished, seldom 

 showed any striae, and usually were not as intensively abraded. In the remaining 

 three animals, the lingual facets were similar in shape to those on the teeth of 



