78 



NORTH AMERICAN FAUNA 74 



% i 



n m m ^' 



l^H g)® • mf 



Fig. 50. Calcified teeth of a fetal Pacific walrus at 6 months post-implantation. Develop- 

 ment of the primary teeth (rows b and c) is nearly complete, whereas only partial crowns 

 of the secondary teeth (rows a and d) have developed. (Photo by G. C. Kelley) 



Fetuses, 450-635 mm, 30 November-16 December. — Eight specimens were 

 dissected and found to be about equally advanced in their dental development. 

 None had the complete primary dentition, but one lacked only m^ and m2. The 

 least number of primary follicles present was 



2-3 

 2-3 



1 



c-j-p 



2-3-4 

 2-3-4 



X 2 



24, 



The follicle of pi was identified in half of the specimens, but in only one of these 

 was there any sign of its being calcified. The development of most of the other 

 primary teeth had progressed to the extent that they had a well-formed crown 

 and root (Fig. 50). The roots of pg and p4 were bifid in some instances, and they 

 and the other primary teeth more closely resembled those of otariids than of 

 phocids (cf. Fig. 50 with Plate 11 of Scheffer and Kraus 1964). The crowns of the 

 secondary teeth already were partly formed. 



Full-term fetuses and newborn calves, 3-26 May. — The primary teeth of these 

 specimens, examined by dissection, were in an advanced state of retrogression, 

 commensurate with the rising secondary dentition, which was already well 

 formed (Fig. 51). Only small remnants of the primary teeth remained, most of 

 which were crowns alone, the roots having been resorbed. In the loci of most of 

 the other primary teeth, only soft tissues or tiny caseous cysts about 1 mm in 

 diameter remained. The minimal formula of calcified remnants was 



