250 MAMMALIAN DENTITION 



M. F. Woodward this condition is brought about by the pres- 

 ence of a large canine which, occupying so much space in the 

 jaw, results in a deformed permanent first premolar and leaves 

 no room for a corresponding milk tooth. This view seems to 

 be borne out by the cases of the Hyrax and the Tapir (see Fig. 

 77) in both of which the first premolar possesses a predecessor. 

 In the former animal the canine is vestigial and never becomes 

 a functional tooth, in the latter it is reduced and separated by 

 a considerable distance from the premolar series. 



As for eruption of the deciduous teeth it is well known 

 that the lower central incisors are the first to come into place 

 in the jaw. When one recalls that in the act of sucking the 

 maternal nipple is compressed between tongue and palate, ths 

 lower jaw serving merely to steady the tongue, it becomes 

 evident that these teeth can erupt without causing irritation 

 or damage to the delicate skin of the nipple and so enable the 

 mother to continue suckling her infant for a longer time than 

 would be possible otherwise. It is also clear that as the young 

 animal grows and the jaws lengthen there will be unoccupied 

 space behind the second milk molar. This provides a site for 

 the developing first molar which is thus the earliest of the 

 permanent teeth to come into position. 



The deciduous dentition of both Old- and New-World Mon- 

 keys affords confirmation of the conclusion attained above that 

 the milk teeth do not resemble in miniature any of the perma- 

 nent set. 



In some Old- World Monkeys, Pithecus rhesus for example 

 (Fig. 92), the differentiation of the first milk postcanine in 

 both jaws has proceeded to a much more molariform stage 

 than in the Anthropoids generally. The upper tooth is quad- 

 ricuspid from the appearance of a hypocone and although the 

 trigonid of the lower tooth is relatively narrow the talonid is 

 comparatively better developed than even in Man. The second 

 milk postcanine is more completely a replica of the first perma- 

 nent molar than in the Anthropoids or in ourselves, a condi- 



