CHAP. Vl] THE DENTAL SYSTEM OF THE PRIMATES 291 



Turning from the molar to the premolar teeth, it may be 

 repeated that Professor Osborn claims that the Theory of Tritu- 

 berculy applies to the lower series, though in regard to the upper 

 premolars, a most important difference has to be admitted. The 

 difference is important because it has been held to throw doubt 

 on the validity of the Theory of Trituberculy. The case of the 

 upper premolar teeth therefore requires special attention. The 

 upper premolar teeth appear to be derived from a conical haplodont 

 tooth similar to the presumed original form of molar tooth. Pro- 

 fessor W. B. Scott 1 observed three stages in the evolution of tin- 

 most complex form of upper premolars. To the primitive cone 

 three cusps are added in succession. They are derived from the 

 cingulum and herein consists the first point of contrast- with the 

 molar teeth, since in the latter the earlier accessory cusps are 

 believed to arise from the sides of the primitive cone or from its 

 base (cf. Fig. 204). With the addition of one cusp the first or 

 bicuspid stage is reached and beyond this we need not pass since 

 this is the ordinary type in the Primates. But the real difficulty 

 of the situation is now to be presented and it consists in the fact 

 that the second cusp (i.e. the first accessory cusp) makes its ap- 

 pearance on the inner side of the primary cone (protocone). But 

 this is exactly contrary to what should happen if the process 

 resembles that claimed for the upper molar teeth on the Tritu- 

 bercular Hypothesis, for ex hypothesi the upper protocone itself 

 should occupy that " internal " relation to the other cusps. But 

 the evidence of embryology is very relevant here, since it testifies 

 that of the two comparable cusps of the upper molar teeth, again 

 the outer is the " older," although again the theory demands the 

 converse. In the tritubercular molar and on the Tritubercular 



1 Proe. Acad. Nat. Si-. Phil. 1892, pp. 405 — 444, quoted by Osborn, Evolution 

 of Mammalian Molar Teeth, 1907, p. 195. 



2 The point is of importance, because there is a difficulty in defining the exact 

 difference between a molar and a premolar tooth. The Horse presents a notable 

 case in point. The older writers seem to have believed that whereas the premolars 

 are preceded by milk teeth, the true molar teeth are not so preceded. Difficulties in 

 the applicability of this criterion having arisen, a difference in developmental history 

 was somewhat eagerly seized upon as an alternative test, but this in turn seems to 

 be a controvertible point, in view of the researches of Marett Tims (cf. J. A. P. 

 xxxvn. p. 146). 



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