56 ODOB^ENUS EOSMARUS ATLANTIC WALRUS. 



unfrequently persistent to extreme old age, although commonly 

 lost in macerated skulls. These rudimentary teeth are usually 

 described as l milk-teeth '; even the posterior ones are some- 

 times so called, but it appears to me an open question whether 

 they do not rather represent permanent teeth in a rudimentary 

 or aborted condition." 



Huxley, in his "Anatomy of Vertebrated Animals" (pp. 300, 

 361), published in 1872, adopts the following as the dental for- 

 mula of the Walruses : I. i=-J, C. =-J, p. m. m. |=| + ^J." He 



says : " The dentition of the Walrus is extremely peculiar. In 

 the adult, there is one simple conical tooth in the outer part of 

 the premaxilla, followed by a huge tusk -like canine, and three, 

 short, simple-fanged teeth. Sometimes, two other teeth, which 

 soon fall out, lie behind these, on each side of the upper jaw. In 

 the mandible there are no incisors, but a single short canine is 

 followed by three similar, simple teeth, and by one other, which 

 is caducous." * Both here and in the formula no reference is 

 made to the deciduous incisors, although the caducous molars 

 are recognized. 



In the foregoing resume, we have seen how vague was the 

 information bearing on this subject possessed by all writers 

 prior to about the beginning of the present century ; IIOAV the 

 earlier notices of the existence of incisors in the young were 

 overlooked and rediscovered by later writers, as well as how 

 slowly the first permanent tooth of the molariform series of the 

 upper jaw came to be generally recognized as a true incisor and 

 not a molar ; how, later, the number of incisors in the young was 

 found to be six in the upper jaw and six in the lower jaw, with, 

 as a rule, two small caducous molars on each side in the upper 

 jaw, and one on each side in the lower behind the permanent 

 grinding teeth; that the first permanent molariform tooth of 

 the lower jaw was a canine and not a molar; and that by dif- 

 ferent writers the number of incisors recognized in the lower 

 jaw has been sometimes four and sometimes six, and the cadu- 

 cous upper molars regarded sometimes as one and sometimes 

 as two. Finally, that the true formula of the full dentition of 

 the Walrus is I. |=|; C. =-\; Pm. M. f5|=g=34. It hence 

 appears that the dentition of the Walruses is peculiar and some- 

 what abnormal in four features, namely, (1) the early disap- 

 pearance of all the incisors except the outer pair of the upper 



* Aiiat. Vertebr. Anim., pp. 3GO, 361. 



