16 ELEPHANTS. 



From the formula it will be seen that in the upper jaw only one 

 premolar is wanting to complete the primitive number, while 

 in the lower jaw an incisor and the canine are missing on 

 each side in addition to the premolar. 



Of the upper incisors the second pair (i. 2) are greatly 

 enlarged and form strong downwardly directed tusks,, the 

 beginning of the great tusks so characteristic of the later 

 elephants. The canine (c.) seems to have been quite small and 

 unimportant, being on the way to disappearance. The pre- 

 molars (pm.) are separated from the canine by a short interval, 

 and as already mentioned the anterior one of the full denti- 

 tion is wanting. The remaining premolars are all simpler in 

 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. 



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

 of the fceeth, 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. 



In the lower jaw (figs. 5 & 7 C) the middle incisors (i 1) are 

 small, the second pair (i 2) large and tusk-like ; both are 

 directed forward and their upper surface continues forward the 

 surface of the spout-like anterior portion of the jaw. The third 

 incisors, the canine, and the first premolar of the full dentition 

 are wanting. The remaining three premolars (pm.} 3 which re- 

 place milk-molars, are simpler than the molars, and only in the 

 third and fourth is there any arrangement of the anterior 

 cusps to form a transverse ridge. The first and second molars 

 (fig. 70), like those of the upper jaw, consist of two transversely- 

 arranged pairs of knobs and a posterior knob which is larger 



