THE TEETH OF VERTEBRATES 



71 



in an antero-posterior line, forming a three- 

 cusped crown called the triconodont form. This 

 is the type of the early forms of the mammalian 

 molar teeth and is still preserved in some of the 

 carnivora, seals, lemurs and other living species. 

 The next stage is the shifting of the cusps so as to 

 alter their relative positions to form a triangle. 

 In the upper arch the protocone shifts to the 



Fig. 19. — Diagram showing the occlusion of the trigon and trigonid. 



a. Protocone (Lingual cusp). 



b. Paracone (Mesio-buccal cusp). 



c. Metacone (Disto-buccal cusp). 



1. Protoconid (Buccal cusp). 



2. Paraconid (Mesio-lingual cusp). 



3. Metaconid (Disto-lingual cusp). 



lingual and becomes the mesio-lingual cusp, leav- 

 ing the paracone and the metacone on the buccal 

 side. The paracone is the mesio-buccal cusp and 

 the metacone the disto-buccal cusp. This is the 

 tritubercular molar crown of early geological 

 times from which all other molars of the present 

 mammals are probably developed. This triangu- 

 lar molar crown is still present in the opossum 

 (Fig. 18), some insectivora, lemurs, and others. 



