614 HISTORICAL GEOLOGY 



lower tusks were much longer than in the earlier form, those of the 

 upper jaw being large, with a slight downward curve. This en- 

 largement of the tusks was accompanied by a decrease in the total 

 number of teeth, and the three-ridged grinding (molar) teeth were 

 better instruments for mastication than those of the earlier elephant. 

 The structure of the skull shows that the upper lip, in this form, had 

 probably been developed into a short trunk which, however, may 

 not have extended much beyond the lower tusks. 



An elephant from the Miocene of France (Tetrabelodon), smaller 

 than the modern Indian elephant, shows a great advance over those 

 of the Eocene. This is to be expected, since no Oligocene elephants 

 have yet been found. In this form the upper tusks are long and 

 almost straight, while the lower are short, but since they are set in a 

 greatly elongated lower jaw, they project almost as far as the upper. 

 The trunk was longer than in the earlier members of the family, 

 resting upon the lower jaw, and could only be raised and moved 

 from side to side. As the trunk lengthened the neck shortened, 

 since the animal could feed without the mouth's reaching the ground. 

 Moreover, as the tusks and trunk became heavier, a long neck would 

 have been a mechanical disadvantage. The teeth in this form are 

 quite large and have numerous elevations and ridges. This genus 

 spread over the Old World and North America, and by the dropping 

 of the lower tusks which permitted the trunk to hang straight down, 

 gave rise to the mastodon (Dibelodon). The mastodon differs from 

 the true elephant in two principal particulars: (i) in the teeth 

 (Fig. 548), which are composed of ridges covered with enamel, while 

 the grinding surface of the elephant's tooth is made up of vertical 

 plates of enamel and dentine, alternating with cement, the mastodon 

 tooth being adapted for crushing succulent vegetation such as leaves 

 and tender twigs, but not for grinding hard grasses, as is the ele- 

 phant's; and (2) in the greater length of the lower jaw which in true 

 elephants is remarkably short. The true elephants were apparently 

 derived from a branch (Stegodon) that lived in India during the 

 Pliocene. In this form the teeth show the first signs of developing 

 cement between the ridges, which, by further development, formed 

 the remarkable grinding apparatus of the mammoth and modern 

 elephant. 



Both the mastodons and the true elephants (mammoths) spread over 

 North America, living here even after the disappearance of the last 

 ice sheet, as is proved by the presence of skeletons in the bogs that 



