XII 



LIFE OF TERTIARY PERIOD 231 



right direction. But here (owing probably to some excep- 

 tionally favourable conditions), after first lengthening both 

 lower jaw and lower tusks to keep pace with the upper 

 ones, a reversal of the process occurs, reducing first 

 the lower tusks, then the lower jaw, till these tusks 

 completely disappeared and the lower jaw was reduced 

 to the most useful dimensions in co-ordination with a 

 greatly lengthened and more powerful trunk. Although 



Fig. 85. — Skeleton of Mastodon americanus. 

 From the Pleistocene of Missouri, U.S.A. Length, 20 feet ; height, 9^ feet. 



(B.M. Guide.) 



in this case the gaps are still rather large, there can be no 

 doubt that we have here obtained a view of the line of 

 development of the most remarkable land mammals now 

 living from a small generalised ungulate mammal, as 

 indisputable and as striking as that of the horses from the 

 little five-toed Eohippus of the American Eocene. 



It may be here mentioned that the huge American 

 Mastodon has been found in the same deposits with stone 

 arrow-heads, and was undoubtedly hunted by early man ; 



