CHAP. VI] THE DENTAL SYSTEM OF THE PRIMATES 287 



(iii) This section of the theory of Trituberculy is admittedly 

 menaced if not actually overthrown. Osborn (1907) 1 maintained 

 that the balance of evidence favoured the Tritubercular Theory. 

 In 1910, Gregory 2 stated that in his opinion the view may still 

 be correct that all the accessory parts are the offspring of a main 

 protocone, the tip of which lies on the inner side of the upper, 

 and on the outer side of the lower molars. 



(iv) Rotation of the lateral denticles. This suggestion has 

 been abandoned through lack of evidence. The objection based 

 upon embryological grounds has been mentioned already. It 

 seems undeniable that embryologically the cusp first developed 

 is not the protocone, as should be the case in accordance with 

 the hypothesis (cf. Taeker, quoted by Osborn, Am. Nat., 1893; 

 Woodward, quoted by Tims, ./. A. P. xxxvu. p. 137; Tims, op. 

 cit. p. 137 ; Heischmann, quoted by Rose, Anat. Ariz. Band VII. 

 p. 394 ; and Rose, ibid.), nor does there seem to be any embryo- 

 logical evidence of the circumduction (often spoken of as the 

 rotation) of the secondary cusps. 



It might thus seem that the objections are directed from the 

 side of embryology only, but Tims (op. cit. p. 138) claims that the 

 evidence of palaeontology is by no means flawless. 



That the triangles of cusps bear a " reversed " relation to each 

 other is however still maintained. Professor Gregory in 1910 

 expressed the opinion that the trigon (or triangle) above and the 

 trigonid below may still be morphologically reversed structures. 



(v) As a matter of observation, this part of the subject has 

 undergone no modification. 



(vi) This conclusion has been notably modified. Instead of 

 making an appeal to the Tritubercular type (as defined above) as 

 the ancestral one, the following terms are substituted (by Professor 

 Gregory, Bulletins of the Geological Society of America, Vol. xxni. 

 June, 1912, p. 192) : " There can now be very little doubt that many 

 or perhaps all placental orders at one time passed through a stage 

 in which the upper molars were trigonal, the lower tuberculo- 

 sectorial." It is noteworthy that this description of upper molars 



1 Evolution of Mammalian Molar Teeth, p. 3. 



2 Bulletins of the American Museum of Natural History, Vol. xxvn. p. 184. 



