DEVELOPMENT OF THE TEETH IN MAMMALIA 41 



that each primate tooth is a double structure equivalent to 

 two reptilian teeth. This view is a compromise between 

 the concrescence and the differentiation theories. 



* In the reptilian ancestor of the Mammalia a triconodont 

 differentiation had arisen, and two such triconodont teeth, 

 one buccal and one lingual, had become fused. The fusion 

 of these two triconodont teeth, the lingual side of one, to 

 the labial side of the other, yielding a six-cusped element as 

 the typical mammalian tooth, the varieties arising by 

 a secondary reduction of this six-cusped tooth.' 



It is seen that Bolk disposes of the assumption that each 

 cusp of the mammalian tooth represents a single reptilian 

 cone-tooth. 



Summary 



To account for the evolution of the human molar tooth, 

 several theories have been formulated, the most important 

 of these being : 



Trituberculism. 



The Polybuny or Multicuspidate theory. 



Concrescence. 



Bolk's triconodont concrescence theory, as it might perhaps 

 be called. 



The Tritubercular theory is a very important generaliza- 

 tion, and although not generally accepted in its entirety, 

 throws great l : ght upon the relations of the cusps and the 

 functional evolution of the human molar. 



This theory, founded by Cope on the discoveries of early 

 mammalian remains in New Mexico, considers that the 

 principal cusps of both upper and lower molars are produced 

 by the addition of cusps to the first-formed protocone, 

 which represents the original haplodont or single cone of 

 the reptiles. These added cusps take the shape of a triangle 

 in early forms of the Mammalia, in the upper jaw the base 

 of the triangle being outwards, the protocone forming the 

 inner cusp ; in the lower molars the base of the triangle is 

 inwards, the protocone forming the outer apex of the 

 triangle. This primitive triangle is called the trigon in the Trigon 

 upper teeth, the trigonid in the lower. To this trigon Trigoni 

 another element is added, the heel or talon, the trigon 



