72 COMPARATIVE DENTAL ANATOMY 



This triangular arrangement of the cusps forms 

 the trigon of the upper molars. 



In the lower molar, the primitive cusp is called 

 the protoconid and moves to the buccal side and 

 becomes the mesio-buccal cusp. The paraconid 

 becomes the mesio-lingual cusp and the metaconid 

 becomes the disto-lingual cusp, thereby forming 

 the trigonid of the lower molar. Thus the tri- 



Fig. 20. — Diagram showing the occlusion of the talonid of the lower 

 molar in the trigon of the upper. 



1. Protoconid (Mesio-buccal cusp). a. Protocone (Lingual cusp). 



2. Paraconid (Mesio-lingual cusp). b. Paracone (Mesio-buccal cusp). 



3. Metaconid (Disto-lingual cusp). c. Metacone (Disto-buccal cusp). 



4. Hypoconid (Disto-buccal cusp). 



angles of the upper and lower molars alternate 

 (Fig. 19), — the apex of the upper one being di- 

 rected lingually and the lower one buccally, — so 

 they pass in a shear-like motion. The next step 

 is the addition to the trigonid (of the lower 

 molar), on the disto-buccal surface, of a heel or 

 talonid, which may support one, two or three 

 cusps. The buccal is called the hypoconid, the 

 disto-buccal the hypoconulid, and the disto-lingual 



