PALEONTOLOGY 



323 



Elephas. — In this genus are included a number of extinct forms 

 (the mammoths) from three or four continents, and the Hving elephants. 

 The extinct forms, though called mammoths, were not large animals, 

 being no larger than the Indian elephant of today, and not so large as 

 the living African species. Some of the features of the elephants, 

 their size, the short neck, the long proboscis, and the heavy tusks are 

 matters of common observation. The skull is very high and short 

 (Fig. 2 18, A')- The height is due chiefly to the development of cancellate 

 bone, not to the enlargement of the brain which is still quite small. 

 As stated above, the high skull affords the necessary leverage for the 

 muscles that support the weight of the tusks. The molar teeth are dis- 

 tinctly grinding teeth (Fig. 218, A; see also Fig. 220). Each tooth bears 

 a number of transverse ridges, about ten in the African elephant and 

 two dozen or more in the Indian species.. These ridges are worn down 



A B 



Fig. 220. — Tooth of Mammoth (Elephas) from the Pleistocene, showing the flat 

 grinding surface and the numerous plates of enamel bound together by cement. A, side 

 view; B, surface view. {From specimen discovered at Ridgeway, Michigan, in 1912, and 

 preserved in the Museum, of Geology, University of Michigan.) 



by the chewing of harsh food, so that the upper surface displays a number 

 of flattened tubular plates of enamel enclosing dentine and bound to- 

 gether by cement. A tooth is completely worn out by use, and is re- 

 placed by another. The method of replacement, however, is peculiar. 

 While the tusks (incisors) are of two sets, one following the other like 

 milk and 'permanent teeth of other mammals, the grinders succeed one 

 another in continuous fashion. There are never more than two visible 

 grinders on each side of each jaw. As they wear out they move forward 

 in the jaw, and are replaced by new teeth appearing behind. New molars 

 thus enter at intervals of two to four years in young elephants, and at 

 intervals of 15 to 30 years in later life. If an elephant lives long enough 

 (60 years or more) it develops a total of 28 teeth, including tusks, but 

 has not more than ten (often less) at any one time. 



Correlated with the nature of the teeth of the elephants is their food 



