216 ANIMAL LIFE PAST AND PRESENT. 



scarcely be mentioned, there are but two species of 

 Elephant, both nearly related, but one being confined 

 to the African Continent and the other inhabiting 

 India and some of the adjacent regions. In the later 

 Tertiary period of the earth's history Elephants were, 

 however, much more numerous, and were then spread 

 over the greater part of the surface of the globe, having 

 been obtained from Europe and Asia, as far north as 

 Siberia, North and South America, and North Africa. 

 Many of these extinct Elephants, and, indeed, all of 

 the earlier ones, differed very remarkably from the 

 living species in the much simpler structure of their 

 teeth ; these species being known as Mastodons, a term 

 which has now become almost a popular one. 



In common with true Elephants, Mastodons differ 

 from other Mammals, in that, instead of having all 

 their cheek-teeth in use at the same time, the hinder 

 ones gradually come up in an arc of a circle behind 

 the tooth in use at any one particular period, which 

 is gradually worn away and shed. Further, the teeth 

 gradually increase in complexity from before back- 

 wards, the most anterior ones in some cases not having 

 more than two ridges (Fig. 74), while the hinder ones 

 are much more complex (Fig. 76). It results from this 

 peculiar mode of succession that there are never more 

 than portions of two, or at most of three, teeth on 

 either side of each jaw in use at any one time. Fig. 

 74 shows that the simple cheek-teeth of a Mastodon 

 are really constructed on the same general plan as 

 those of a Pig (Fig. 65), the outer cones having more 



