DENTITION. 5 



o 



Professor Owen, in referring to instances of deviation from this 

 formnla, dependent on differences of age and sex, stated ''that 

 occasionally a small tooth was fonnd anterior to the normal series 

 of four, and more commonly in the upper than the lower jaw ; and 

 that, more rarely, a small tooth was superadded behind the nor- 

 mal four, in the upper jaw, and still more rarely in the lower 

 jaw ', the formula of the dentition of such varieties, in excess, 

 being, 1.|^2> ^- J~S? P. |^, M. j^J=26." Owen here makes 

 no reference to the literature of the subject, and evidently gave 

 a very erroneous interx^retation of the dental formula. In his 

 later references to the subject he gives an entirely different in- 

 terpretation, and one more nearly agreeing with that now com- 

 monly accepted. In his latest reference to the subject,* he 

 writes : " In the Walrus ( Tricliechus rosmarus) the normal incisive 

 formula is transitorily represented in the very young animal, 

 which has three teeth in each premaxillary and two on each 

 side of the fore part of the lower jaw ; they soon disappear 

 except the outer paii* above, which remain close to the maxil- 

 lary suture, on the inner side of the sockets of the enormous 

 canines, and commence the series of small and simple molars 

 which they resemble in size and form. In the adult there are 

 usually three such molars on each side, behind the permanent 

 incisor, and four similar teeth on each side of the lower jaw; 

 the anterior one passing into the interspace between the upper 



incisor and the first molar The canines are of enormous 



size Their homoty]_)e below retains the size and shape of 



the succeeding molars." The formula of the normal dentition 

 apparently here recognized is : 1. 1^ ; 0. J^J j M. |5|=j|=26. 

 Giebeljt in 1855, gave six incisors both above and below as the 

 number existing in the young before and for a short time after 

 birth. Of these, the lower are said to soon fall out, their alveoli 

 then becoming fiUed with a bony deposit. Of the upper inci- 

 sors, the inner pair first disappear, and soon after them also the 

 middle pair, leaviug only the outer pair, which begin the mola- 

 riform series, and to which they are often referred, this outer 

 pair persisting till late in life. The upper canines, he says, are 

 never cast. | In the lower jaw, the first permanent tooth is 

 regarded as a canine, because it is thicker and rounder than the 

 posterior teeth, and lacks the cross-ftirrow that marks the oth- 



* Anatomy of Vertebrates, vol. iii, p. 338. 

 tOdoutog., p. 82,. pi. 36, fig. 7; Saugeth., p. 129. 



X They are, however, as shown by Malmgren (see beyond), preceded in the 

 embryo by temj^orary teeth. 



