290 THE DENTAL SYSTEM OF THE PRIMATES [SECT. A 



But comparison with the " tuberculo-sectorial " type of lower 

 molar as shewn in Fig. 203 shews that the human lower molar is 

 very closely similar to that type, which (as noted above) is regarded 

 as an " ancestral " one. 



Similar remarks apply to the other Simiidae, to the Primates 

 generally, and to Tarsius especially. 



One prominent fact brought out by the study of these teeth 

 as carried out on the lines suggested above, is that supple- 

 mentary parts (hypocone above and talonid below) are appended 

 to a triangle of cusps. The significance of these appendages 

 seems to find an explanation when the teeth are shewn in adjust- 

 ment. When the upper and lower teeth are thus adjusted, as 

 shewn in Fig. 207 D, it will be seen that the " talonid " of the 

 lower molar extends beneath the protocone of the upper tooth, 

 while the hypocone or upper appendage comes down upon the 

 primitive lower triangle or trigonid. At this stage we must 

 remember the important point made in connection with the Tritu- 

 bercular theory as first stated, viz. that the primitive triangles 

 alternate in the upper and lower jaws. The appendages just 

 described (viz. hypocone and talonid) represent the means by 

 which the vacant spaces (alternately above and below) are filled 

 so that the whole apparatus becomes a more efficient instrument. 

 In fact it meets the need imposed by the change in function 

 of the jaws from a comparatively simple prehensile apparatus to 

 a mechanism for fine trituration and division of the food prior to 

 its being swallowed. The origin of all parts accessory to the 

 primitive cone is almost certainly dependent upon that need and 

 the method of its realization. The precise manner in which the 

 transformation has been effected is as yet unknown, but the 

 "wedge-theory" proposed by Dr Gregory in 1910 1 provides at 

 least a basis for research and a reasonably suggestive sketch of 

 what has actually occurred. This brief statement must suffice 

 here, since detailed discussion of the mode of origin of accessory 

 cusps is precluded in this place. In conclusion, I may point out 

 once more that the molar dentition of the Primates including 

 that of the Hominidae, must be placed in a position close to that 

 of the type claimed as distinctive of the ancestral mammals. 

 1 Bull. Am. Mns., 1910, pp. 184 et seq. 



