THE TEETH OF VERTEBRATES 59 



i.e., the first incisor to the left of mesial line and 

 above fraction line. The same tooth below would 

 be ifTT The right upper first molar 1 m\, the lower 

 left second molar |m 2 , the right upper second pre- 

 molar 2 pm |, the left lower third molar |m 3 ; the 

 number being distal of the initial letter and the 

 line mesial. 



The classes of the teeth in Mammals. 



The Incisors take their name from the office 

 they perform in the function of mastication, — i.e., 

 to incise, to cut, but the term is applied to the 

 upper teeth located in the premaxillary bone an- 

 terior to the intermaxillary suture, whatever their 

 form, in mammals. The teeth in the mandible 

 which occlude with the upper teeth (in the pre- 

 maxillary bones) are also incisors. Thus the 

 tusks of the elephant are incisors, although their 

 cutting function is completely aborted and these 

 teeth are employed as tools and weapons only. 

 The function of cutting or dividing food is per- 

 formed by various organs throughout the animal 

 kingdom. Teeth for this purpose are developed 

 very low in the scale of life, as the cephalopods 

 have cutting teeth; the sea-urchin has highly 

 specialized incisors ; the insects and worms cut by 

 means of the mandibles, jaws, etc. The beaks of 

 turtles and birds are employed as cutting imple- 

 ments, but these are not true teeth. The fishes 

 and reptiles have no true incisors, as the teeth are 



