74 



NORTH AMERICAN FAUNA 74 



Burns and several larger fetuses and older animals that I obtained at Gambell. As 

 a result of that investigation, I was able to identify several teeth that had not 

 been recorded previously in the walrus and to contribute to solution of some of 

 the previously controversial problems. 



The specimens examined were 15 fetuses, ranging in age from about 1.5 to 6 

 months post-implantation, 38 calves and full-term fetuses, and 135 other animals 

 that were from 1 to more than 30 years old. Of the smallest fetuses, seven were 

 stained with alizarin red S, macerated in potassium hydroxide, and cleared and 

 mounted in thermal plastic or in glycerol. In three of these, only half of the head 

 was stained and cleared; the other half was serially sectioned. The dentitions of 

 two other fetuses and four calves were examined roentgenographically in lateral 

 oblique and ventral views and, subsequently, by dissection. The dentitions of 

 four fetuses, nine calves, and 15 older animals were studied by gross dissection 

 and subsequent examination of the cleaned, dried skulls. Other specimens, 

 examined in the field, were 60 cleaned and dried skulls and 78 fresh heads with 

 the soft tissues still intact. In the fresh specimens, I recorded only the gingivally 

 erupted teeth. 



In young fetuses, up to 6 months post-implantation, the presence of a dental 

 follicle, alveolus, or both was accepted as evidence of a developing tooth, even 

 when calcification was not detected. The "dentitions" of such specimens are 

 described by a "follicle formula," to distinguish them from the actually calcified 

 teeth of older specimens. The dental formulae of the older specimens include 

 only those teeth that were represented by calcified tissue. 



Primary Teeth 



The formula for the complete primary dentition of the walrus is 

 . 1-2-3 1 1-2-3-4 1-2 ^ _ 



^1:1:3 ^-P-^:3T"^TT^' = ^^- 



None of the specimens that I examined had this full complement; the maximum 

 in any one specimen was 34 teeth, lacking only m2 and mQ. However, these 

 "missing" molars were found in other specimens whose dentitions were 

 incomplete in other respects. My designation of the lower premolars here as 

 2-3-4, rather than 1-2-3, is new and is based on my observations that (1) they are 

 the only primary postcanine tooth follicles that are succeeded by secondary teeth, 

 (2) all three develop into relatively stout, well-calcified teeth, as do P2-3-4 (but 

 not pi) in other pinnipeds, and (3) the relative size, location, and occlusal pattern 

 of the secondary teeth that succeed them also correspond to those of P2-3-4, rather 

 than of Pi.2-3- The supporting evidence for these criteria is discussed below in 

 Comparative Dental Formulae. 



The ontogeny of the primary dentition, as seen in my series of fetuses was as 

 follows: 



Fetiis, 106 mm, 11 August (stained and cleared). — Shallow depressions in the 

 alveolar bone of the upper and lower jaws indicated the presence of follicles of 



. 2-3 1 1-2-3-4 . . . 



^— ^-P^TT^ ^ = 



Some of these can be seen in Fig. 48 (left). The palate of this specimen was 



