9 2 



GIDLEY 



Of the several theories thus far advanced for the evolution of 

 the teeth, none has been entirely satisfactory, and there is still 

 a wide disagreement among authorities, especially as regards 

 the position of the primary cone or " protocone " in the upper 

 molars. As proposed by the late Prof. E. D. Cope and sup- 

 ported by Prof. Henry F. Osborn, the primary cone is to be 

 found invariably on the inner or lingual side of the trigonodont 

 upper teeth, and is the homologue of the central cone in Tri- 

 conodon, in which the three main cusps are arranged in an 

 antero-posterior line, the trigonodont molar having been derived 

 from this form through the shifting of the two lateral cones to 

 the outside. The central cone {protocone) remaining on the 

 inner side, thus forms a triangle {trigori) with the apex pointing 

 inward. In the meantime, according to this theory, the cusps 

 of the lower molars are supposed to have moved in the opposite 

 direction, leaving the central cusps (flrotocoui'd) on the outside, 

 forming an oppositely directed triangle (trigonid). Thus the 

 primary cones of the upper and lower molars in shifting have 

 completely reversed their positions in relation to each other, the 

 primary cone of the upper molars not only moving to the inner 

 side of the crown, but taking a position in the series inside the 

 primary cone of the lower molars as well. 



This theory, so skillfully worked out by Osborn, has been 

 widely accepted as satisfactorily explaining the problem of tooth- 

 cusp evolution. But recent paleontological and embryological 

 investigations have thrown a large amount of discredit on the 

 whole theory. As stated by Wortman, Scott has shown most 

 conclusively, from paleontological evidence, that in the upper 

 molariform premolars the primary cone is on the outer side and 

 the subsequently added cusps have a very different history from 

 that proposed by the tritubercular theory for the true molars. 

 The embryological researches of Woodward, Tacker, and others 

 have not only confirmed Scott's theory for the premolars, but 

 show also that in all groups of mammals investigated the antero- 

 external cusp or paracone is first to appear in the permanent 

 upper molars and milk molars, as it does in the premolars, and 

 the order of appearance of the other principal cusps is practi- 

 cally the same as proposed by Scott for the premolars. 



