THE SKELETON IN MAMMALIA. THE TEETH. 435 



The last three represent the true molars of ordinary mammals, 

 those in front appear to be milk molars which are never re- 

 placed by permanent successors, but the whole series gradually 

 moves forwards in the jaw, and the teeth become worn away 

 and their remnants cast out in front while development of 

 others proceeds behind. The individual teeth are so large 

 and the processes of growth and destruction by wear take 

 place so slowly, that not more than one or portions of two 

 teeth are ever in place and in use on each side of each jaw 

 at one time, and the whole series of changes coincides with 

 the usual duration of the animal's life. On the other hand 

 the Dinotherium, the opposite extreme of the Proboscidean 

 series, has the whole of the molar teeth in place and use at 

 one time, and the milk molars are vertically displaced by pre- 

 molars in the ordinary fashion. Among Mastodons trans- 

 itional forms occur in the mode of succession as well as in 

 structure, many species showing a vertical displacement of 

 one or more of the milk molars, and the same has been ob- 

 served in one extinct species of Elephant (E. planifrons) as 

 regards the posterior of these teeth." 



In the TILLODONTIA the grinding teeth are of Ungulate 

 type, while the second incisors are large and grow from per- 

 sistent pulps, so as to resemble those of Rodents. 



BODKNTIA have a most characteristic and very constant 

 dentition, the common dental formula being 



* r G l pm (=1) m l> total 18 or 2a 



The incisors always have chisel-like edges and persistent 

 pulps, and are separated by a wide diastema from the 

 premolars. Canines are always absent, and there are ge- 

 nerally three grinding teeth not preceded by milk teeth ; 

 their surface may be grooved, or may be bunodont. Teeth 

 are most numerous in the Duplicidentata (Hares and Rabbits), 



9 A 33 

 in which the formula is i y c -pm-m^, total 28, and fewest 



282 



