338 



PRINCIPLES OF ANIMAL BIOLOGY 



distinct transverse ridges, though not many of them, and its lower jaw 

 was short. 



The extinct elephants known as mammoths belong to Pleistocene 

 time, while from them or directly from Stegodon have arisen two kinds 

 still living, the Indian and the African elephant. The gross features of 

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

 The skull is very high and short (Fig. 283A'), due chiefly to the develop- 

 ment of cancellate bone. As in the earlier forms, the high skull affords 

 the necessary leverage for the muscles that support the weight of the 

 tusks. The molar teeth are distinctly grinding teeth (Fig. 283A ; see 

 also Fig. 285). Each tooth bears a number of transverse ridges, about 

 10 in the African elephant and 24 or more in the Indian species. These 



A B 



Fig. 285. — 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 Ridgeivay , Michigan, in 1912, and preserved 

 in the Museum of Geology, University of Michigan.) 



ridges are worn down by the chewing of harsh food, so that the upper 

 surface displays the cross sections of a number of flattened tubular 

 plates of enamel enclosing dentine and bound together by cement. 

 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. As the molar teeth that appear first 

 wear down they move forward in the jaw and are replaced by others 

 from behind. Three permanent molars may thus successively appear 

 on each side of each jaw, but the wearing and movement are slow, so 

 that the interval between the appearance of the second molar and that 

 of the third may be 30 years. The total number of permanent teeth, 

 including the tusks, is 14. 



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

 and chewing habits. The an(;estral forms whose molars bore prominent 

 elevations lived on twigs and tender herbage which they crushed in 

 mastication, but the mammoths with their flattened tooth surfaces 



