626 CnORDATA. 



lUGiitiil basis the molars are divided into premolars (bicuspids of 

 dentists), which appear in both dentitions, and the true molars, 

 which occur only in the last. 



From the foregoing it will be seen that every species of mam- 

 mal is cliaracterized by its dentition, and these features may be 

 expressed by a short formula. It is only necessary to place the 

 number of each of the four kinds of teeth mentioned in their regular 

 order, those of the upper jaw separated from tliose of the lower by 

 a horizontal line, to express this. Since the two sides of the body 

 are symmetrical, only those of one side need be enumerated, and 

 in case that one kind be absent the deficiency is indicated by a 

 zero. The dental formula of man would thus be f i-||; of the rein- 

 deer, in which in the upper jaw incisors and canines are absent, 

 i'i|-|. The different formula', by comparison, give us a funda- 

 mental formula from which they have been derived by reduction. 

 This was probably ^Y^^- 



The molars undergo, according to the food, the greatest modification 

 of form. As a starting point tlie bunodont tooth may be taken which 

 occurs in omnivorous mammals and which has tlie crown with several 

 blimt projections or cones. With animal food (fig. 650, 657) the cones be- 

 come sharper and cutting (secodont dentition of carnivores and insec- 

 tivores), and when the cutting angle becomes very sharp, with a special 

 prominence on the inner side, it is spoken of as a tlesh or carnasial tooth. 

 In vegetable feeders the cones become connected by crests (lophs) or are 

 half-moon-shaped (lophodont or selenodont). Since the cones and lophs 

 become in part worn away and the grooves between them are filled with 

 cement, there arise broad grinding surfaces strengthened by the harder and 

 more resistant enamel of the cones and lophs ; this extends inwards as 

 fulds from tlio outer enamel wall of the tooth ; the folds may become cut 

 olf and form islands of enamel on the grinding surface (dentes oomplicati 

 of ungulates). When the folds extend in regular order from the outside 

 and inside and meet in the middle they form numerous successive leaves, 

 bound together by cement (compound teeth of elephants, fig. 667, and 

 many rodents). 



Paleontological investigation, with which the more recent embryologi- 

 cal results are in accord, have sliown that a great regularity prevails in 

 the formation of the cones of the molars. Triconodont and tritubercular 

 teeth are recognized, in which the three cones are either arranged in a 

 line or in a triangle, as well as multitubercular teeth with more numerous 

 cones irregularly arranged. The triconodont type develops farther by the 

 formation of secondary cones. The development of these occurs in dif- 

 ferent ways in molars and premolars. Since the latter are the more sim- 

 ple, their distinction from the moliu's does not rest alone upon the existence 

 of a milk dentition, but upon structure as well. This is important, because 

 it happens that there arc premolars which are not replaced (marsupials, 



