16 



ELEPHANTS. 



structure than the molars behind them, and consist of three main 

 cusps only, the two front cusps being arranged in a transverse 

 line in the third and fourth premolars. These teeth are preceded 

 by milk-teeth which they displace from above as in the ordinary 

 mammals ; in the later elephants we shall see that this usual 

 replacement of milk-teeth by premolars is gradually lost. 



Fig. 7. 



Upper and lower teeth of Moeritherium. 



A. Upper teeth. B. PremaxiHa, large tusk-like second incisor. C. Man- 

 dible from other side, c., socket of canine ; i. 1-3, incisors ; m. 1-3, 

 molars ; pm. 2-4, premolars. \ nat. size. 



The molars (fig. 7. A, m.) are the most interesting and im- 

 portant of the teeth, because it is in them that the most nearly 

 complete series of gradually more and more complicated forms 

 can be traced. In Moeritherium the crown of each upper molar is 

 composed of two transversely arranged pairs of knobs, giving 

 rise to a pair of transverse crests ; there are also in many cases 

 small posterior knobs the first trace of the tendency to increase 

 the number of transverse crests by additions to the back 

 of the tooth, which is characteristic of the whole group. 



