TEETH AND THEIR VARIATIONS. 217 



or less completely united with the inner ones to form 

 two transverse ridges. In some of the earlier teeth of 

 the Mastodon there may be only two such ridges (Fig. 

 74), but in the later ones the number of ridges is gene- 

 rally either three or four 

 (Fig. 75), with additional im- 

 perfect ridges at the two ex- 

 tremities. In such a tooth 

 (Fig. 75), it will be seen that 

 the transverse ridges are low 

 and roof-like; the valleys 

 separating one ridge from 

 another being broad and com- 

 paratively Shallow, Without FIG - ' 4 --An anterior cheek-tooth of 



a Mastodon. 



any of the substance known 



as cement at their bases. When the enamel on the 

 summit of such ridges is worn through by the abrasion 

 of the teeth of the upper jaw against those of the lower, 

 oval or trefoil-shaped islands of the underlying dentine 

 are revealed, as in Fig. 76. The latter figure exhibits the 

 last tooth of a Mastodon in which there are five com- 

 plete transverse ridges, this being the most complex 

 form of tooth found in any of the Mastodons. 



In certain Tertiary rocks in India lying along the 

 southern flanks of the great range of the Himalaya 

 there are, however, found enormous numbers of teeth of 

 peculiar species of Elephants, which, while resembling 

 in many respects those of the Mastodons, have a con- 

 siderably greater number of ridges. Moreover, the 

 ridges themselves are relatively narrower and taller, 



