286 



THE DENTAL SYSTEM OF THE PRIMATES [SECT. A 



series when opposed, while the upper series as a whole are slightly 

 external to the lower set. This condition has been termed 

 anisognathism. 



(vi) The final generalization laid down that the tritubercular 

 type was ancestral to many if not to all the higher types of molar 

 teeth. 



Should the foregoing statement be accepted as a fair expo- 

 sition of the theory of Trituberculy, its rejection clearly cannot be 

 accomplished by the disproof of any single one of its sections. 

 It is reasonable therefore to enquire into the position of each of 

 these at the present time. They will be reviewed accordingly in 

 the same order as that in which they are stated above (p. 283). 



Pro 



-Mo 



A B 



Fig. 206. The upper and lower molars of Dryolestes, an extinct and primitive 

 mammal. The upper molar is drawn with a faint contour. The original cone Pr 

 of the upper molar tooth occupies an "internal" position with reference to its two 

 subsidiary cusps (Pa and M), while in the lower tooth the original cone is now 

 external. Upper and lower teeth alternate. (After Gregory.) 



(i) This is a matter of observation, and as regards the Basal 

 Eocene Amblypoda it still holds good, even though those mammals 

 are not now accorded the " ancestral " status that Cope would 

 have assigned to them. As a matter of fact the most ancient of 

 known mammals (the Upper Triassic Microlestes) is not " Tri- 

 tubercular " but " Multitubercular " in respect of its molar teeth 

 (Fig. 208). 



(ii) The evolution of the molar teeth from a simple conical or 

 haplodont precursor still appears likely, though the haplodont 

 stage in mammalian ancestry must be anterior to that represented 

 by the Thcriodont reptiles, and in point of time must fall in the 

 earlier part of the Triassic period. 



