THE TEETH OF VERTEBRATES 



75 



Evolution of Occlusion. The teeth have been 

 evolved from a single cone to the more complex 

 types and likewise the arrangement of the teeth 

 have changed. In the fishes we find only an oc- 

 clusion of tooth-bearing surfaces. There may be 

 teeth on the maxillae and mandible which occlude 

 with each other as a mass. There may be teeth 

 on the palates and vomer which occlude with 



Fig. 23. — Diagram to show the relation of the upper first and second 

 molars and the lower first and second molars with the cusps named. 



1. Protoconid. 



3. Metaconid. 



4. Hypoconid. 



5. Hypoconulid. 



6. Entoconulid. 



a. Protocone. 



b. Paracone. 



c. Metacone. 



d. Hypocone. 



masses of calcified substances on the tongue. 

 This arrangement is called occlusion of tooth- 

 bearing surfaces, an example of which is seen in 

 the myliobatis (Fig. 24). 



The next step is the occlusion of rows of teeth. 

 In the non-poisonous snakes, as the python (Fig. 

 25), we find two rows of teeth above and one be- 

 low. The lower row occludes between the upper 



