DENTITION. 53 



Professor Owen, in referring to instances of deviation from this 

 formula, dependent on differences of age and sex, stated "that 

 occasionally a small tooth was found 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. 1=|, C.J-5-J, Pm.|=|, M. j^=26. Owen here makes 

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

 a very erroneous interpretation 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 ( TrichecJms rosmarus) the normal incisive 

 formula is transitorily represented in the very young animal, 

 which has three teeth in each prernaxillary and two on each 

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

 except the outer pair 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 homotype below retains the size and shape of 



the succeeding molars." The formula of the normal dentition 

 apparently here recognized is : I. |=J; C. ~(-j M. E|=^=26. 

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

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

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

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

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

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

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

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

 never cast. J 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-furrow that marks the oth- 



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

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



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

 embryo by temporary teeth. 



