ECOLOGY AND BIOLOGY OF THE PAGIFIC WALRUS 83 



molar. It attained the status of a fully calcified tooth in less than half of my 

 specimens, although it always was present in the fetal series as a well- 

 differentiated tooth germ. Its calcification and eruption followed in sequence 

 with the other secondary premolars. Whereas P4 was gingivally erupted in about 

 one-fourth of the 3- to 30-year-old specimens that I examined, it was only lightiy 

 abraded and never was large enough to occlude with an opposing tooth. 



The frequency of occurrence of each of the secondary teeth in my specimens is 

 shown in Table 10. 



Ontogeny and Phylogeny of Canine Migration 



Two unique characters of the walrus' dentition are: (1) reduction in size and 

 posteromediad migration of C^, and (2) radical enlargement and posterolaterad 

 migration of C^ to a position well outside the postcanine row. In my series of 

 fetuses, both of these processes seemed to have been completed about 3 months 

 post-implantation (about 8 months before birth), which implies that these are 

 characters of long standing in the phylogeny of Odobenus. The fossil record 

 seems to confirm that these changes took place long ago. The Miocene walrus 

 Aivukus cedrosemis showed some enlargement of the upper canines, but both the 

 upper and lower canines were in the "usual" canoid positions (Repenning and 

 Tedford 1977). However, some greater advances toward the conditions in 

 Odobenus were apparent in the early Pliocene walrus Prorosmarus alleni, which 

 had large, somewhat laterally placed tusks, and its lower canines were reduced 

 in size (Berry and Gregory 1906; C. E. Ray, personal communication). In the 

 later Pliocene Alachtherium, the lower canine was greatly reduced in size and 

 situated well back from the anterior end of the jaw; the upper canine had 

 become a sizable tusk located lateral to the postcanine tooth row (DuBus 1867; 

 Hasse 1909). 



Comparative Dental Formulae 



The problem of assigning practical descriptive notations to the highly variable 

 dentitions of the walrus was addressed most recently by Cobb (1933), who 

 devised a series of three formulae for the deciduous, successional, a.nd functional 

 teeth. These were intended to describe the typical dentitions of fetal, juvenile, 

 and adult animals, respectively. Although that approach has some merit, it is not 

 the conventional one for dyphyodont mammals. The customary notations are of 

 formulae for the "deciduous" and "permanent" dentitions. The formula of the 

 deciduous teeth comprises the primary incisors, canines, and premolars, which 

 are shed at or soon after birth. The permanent teeth are the secondary incisors, 

 canines, and premolars and the primary molars, which become the functional 

 teeth of the adult. 



By examination of serial histological preparations of the developing fetal 

 dentition, I was able to determine that the primary dentition of the walrus 

 includes tooth germs of at least three more pairs of teeth than were indicated by 

 Cobb's (1933) findings from studies of prepared museum skulls. These additions 

 were pi, m2, and m2, none of which ordinarily is evident as a calcified tooth in 

 cleaned skulls, though each did occasionally reach that stage of development in 

 my series. 



