434 THE VERTEBRATE SKELETON. 



from the mouth. In living forms they are confined to the 

 upper jaw, in some species of the extinct Mastodon however 

 they occur in the lower jaw also. In Dinotherium they are 

 probably absent from the upper jaw, but form a pair of 

 downwardly and backwardly-directed tusks growing from the 

 elongated symphysis of the mandible. 



The grinding teeth in the various Proboscidea show a very 

 remarkable series of modifications. In Dinotherium they are 

 bilophodont or else are marked by three straight transverse 



. ? 23 

 ridges. I he dental tormula is * c -pm-?n~ , and the 



teeth have the normal method of succession. In Masto- 

 don as in Dinotherium the grinding teeth are marked by 

 transverse ridges, but the ridges are subdivided into conical or 

 mammillary cusps, and similar cusps often occur between 

 the ridges. These cusps are covered with very thick enamel 

 and the spaces between them are not filled up with cement. 

 There are six of these grinding teeth for each side of each 

 jaw but only three are in place at once. The first three are 

 milk teeth as they may be succeeded vertically by others. 



In the true Elephants the number and depth of the enamel 

 folds is much increased, and the spaces between the folds are 

 filled up with cement. A very complete series of extinct 

 forms is known with teeth intermediate in character between 

 those of Mastodon and those of the Mammoth and living 

 elephants. The dental formula of Elephas is 

 ,.1.1 7 34 3 

 d *0 % e dm 3=4 m 3' 



Sir W. H. Flower describes 1 the mode of succession of teeth 

 in Elephants as follows : " As regards the mode of succession 

 that of modern Elephants is as before mentioned very peculiar. 

 During the complete lifetime of the animal there are but six 

 molar teeth on each side of each jaw with occasionally a rudi- 

 mentary one in front, completing the typical number of seven. 

 1 Encyclopedia Britannica, article Mammalia, p. 424. 



