446 MAMMALIAN ANATOMY 



positions on the principal cusp, and ultimately in some groups equal 

 it in dimensions ; as in the case of the inferior sectorial of Carnivora 

 and in quadritubercular types." 



(3.) The secondary cusps of the triconodont molars, the paracones, 

 metacones, paraconids, and metaconids, "serve to fill up the spaces 

 between the alternating principal cusps, the protocones and proto- 

 conids, hence a certain amount of interference results. 



" As the lesser cusps are the least resistant to the wedging pressure 

 of such contact, their position would change under its influence, rather 

 than the large central cusps. The lower jaw fitting within the upper, 

 the effect of the collision between the major cusps of the one jaw and 

 cusplets of the other would be to emphasize the relation still more ; 

 that is, the cusplets of the upper jaw would be wedged outward, the 

 major cusps retaining at first their original alternate position (Fig. 

 370, B) . With increase of the size of the teeth the cusps would soon 

 assume in each jaw a position more or less transverse to that of the 

 other jaw, producing as a result of the crowding a crown with a 

 triangular section in both." In other words, in the upper tooth the 

 protocone begins to move inward or medially, the paracone and meta- 

 cone remaining upon the outer side of the crown ; while in the lower 

 teeth the protoconid remains upon the outer side of the crown, the 

 paraconid and metaconid shifting to the inner side; hence in the upper 

 tooth the protocone forms the medial angle of the triangular crown 

 or trigon, but in the lower tooth the protoconid forms the outer angle 

 of the triangular crown or trigonid (Fig. 370, C). This type of cheek 

 tooth is termed the tritubercular type. 



(4.) In the tritubercular dentition the crowns proper of the teeth 

 of one jaw alternate with those of the other (Fig. 370, E). In the 

 next stage of molar development, to the lower molar is added at first 

 a low cingulum at the posterior base ; this gradually develops by use 

 into a broad heel which opposes such parts of the crown of the next 

 upper molar as come in contact with it, and hence to the primitive 

 shearing action between the triangular teeth is added an opposing 

 grinding action, so that each lower molar opposes two upper molars 

 or shears against one and opposes the other (Fig. 370, F). The 

 heel of the lower molar may develop two cusps, whereof the outer 

 is known as the hypoconid and the inner as the entoconid. This 

 type of tooth is known as the tuberculo-sectorial type. From this 



