140 STRUCTURE OF THE VERTEBRATES 



become heterodont, the front being differentiated from the back. 

 In the higher (Therapsid) types there are front incisors, long 

 canines, and primitii^e molars posteriorly. The evidence indicates 

 that the number of replacing teeth was also reduced. 



The mammals show the evolution of the teeth from the simple 

 chfferentiated condition to the highly specialized condition found 

 in many orders. The primitive mammals had molars with three 

 points or cusps. In time accessory cusps developed, and so stable 

 and definite are these cusps that the evolution of the different 

 orders can be traced by their tooth structure. 



The mammals have, two sets of teeth, the baby or milk teeth, 

 and the permanent teeth which replace the first. The posterior 

 molars are an exception to this rule. Those which are replaced 

 are: (1) the incisors which are borne on the pre-maxillary bones 

 of the upper jaw, and the homologous teeth of the lower; (2) the 

 canines, the first tooth on the maxilla of the upper jaw, and the 

 lower tooth which fits in front of it; and (3) the premolars, 

 posterior to the canines, and having more than one cusp in the 

 normal condition. (The Cetacea and some other animals are ex- 

 ceptions to this rule.) Posterior to the premolars are the per- 

 manent molars, which are not replaced after the original erup- 

 tion of the tooth. The difference between premolars and molars 

 often cannot be determined without this knowledge of develop- 

 ment. 



Dental Formula refers to the written formula expressing the 

 number of different teeth in any particular species. In these 

 formulae I represents incisor; C, the canines; P equals the pre- 

 molars, and M the molars. One side of the jaw is given, so that 

 the formula is multiplied by two. The upper teeth are placed 



2 1 2 



above a line, the lower below it, thus: I — -^;C — -ir; P — -o,* 



3 



]\/[ . This is the human formula, and when the numbers are 



o 



added and doubled, it gives the normal thirty-two teeth of the 

 species. 



3 14 3 



The primitive dental formula of the placental is: _; _; _; - 



o i 4 o 



— 44 teeth in all. More specialized animals have many varia- 



\ 



