80 



NORTH AMERICAN FAUNA 74 



The largest primary teeth, other than the molars, were i^, p2, p2, and the 

 upper and lower canines. Some of these also were gingivally erupted. In 28 

 calves examined only for gingival eruption of primary teeth, 9 had c^ erupted, 3 

 had both cj and p2 erupted, 1 had only p2 erupted, and 15 had no erupted teeth. 

 Gingival eruption of primary teeth usually constituted the merest penetration of 

 the gingiva, to the extent that the tip of the crown was barely visible. None of the 

 primary teeth was functional. 



Older animals. — Most of my specimens that were more than 1 year old had no 

 primary teeth other than the molars. Exceptions were a few 2-year-olds which 

 retained one or two primary teeth, such as an upper incisor, an upper or lower 

 canine, or p2; one male that was more than 18 years old still bore both of its 

 primary upper canines. Evidendy, the molars usually are retained permanendy, 

 but they rarely become gingivally erupted and never occlude with an antagonist. 



In summary, my findings indicate that most of the primary tooth germs 

 already are well organized in the fetus within about 1.5 months post- 

 implantation, and that calcification centers of all but the molars appear soon 

 after, within the second month. Their development appears to be completed 

 about 5 or 6 months post-implantation (about 5 to 6 months before birth), 

 whereas full development of the molars is attained much later, about the time of 

 birth. 



Evidently, c^ is displaced posterolaterad vers- early in this development, 

 beginning in the second month. It reaches its ultimate position lateral to pi about 

 3 months post-implantation. Whether the postcanines migrate anteriad at the 

 same time is not clear. In the lower tooth row, the displacement of c^ posteriad 

 into the normal position of pi seems to take place in cadence with the migration 

 of the upper canine. This may be complemented by anteriad migration of the 

 premolars, closing the diastema between c^ and p2 about 3 months post- 

 implantation. 



Each of the primary teeth is much smaller and more slender than its successor, 

 is weakly rooted in a shallow depression in the alveolar bone, and has a simple 

 crown and usually a single root. The primary teeth appear to reach their peak of 

 development about the time when the secondary teeth begin to calcify. The\- are 

 retrogressive at the time of birth and usually are resorbed in place rather than 

 shed, whether or not they are succeeded by a secondary tooth. The frequency of 

 their occurrence in the aggregate of my specimens was as shown in Table 9. 



Secondary Teeth 



The formula for the complete secondar\- dentition of the walrus is 



li±3 ]_ 1-2-3-4 



^ 1-2-3 ^ 1 ^ 2-3-4 ^ - ^'J- 



The secondary molars occur only as undifferentiated laminae in the embryonic 

 dentition and do not to my knowledge ever attain the status of calcified teeth. 

 None of my specimens possessed the full complement of calcified secondar\' teeth, 

 but several fetuses had a full complement of secondary tooth germs. 



Development of the secondary tooth germs begins at least as early as the 

 second month post-implantation, and the tusk (C^), seems to be the first to 

 calcify. In the 178-mm fetus, the tip of the tusk was already partly calcified, 



