MOLAR TEETH. 



worn away and finally discarded. The hinder ones of 

 these teeth (Fig. 6) are so large that while the front 

 portion is being worn away, the back is still bedded in the 

 gum. In the earlier elephants, or mastodons, these molar 

 teeth are composed of a series of relatively low and widely 

 separated transverse ridges, more or less completely divided 

 into inner and outer moieties, and with large open valleys 

 between them. Moreover, in all the teeth, except the last, 

 these ridges do not exceed four in number, although in 

 the sixth tooth (Fig. 4) there may be as many as five or 

 six. Such a molar tooth can be easily derived from the 

 ordinary type of tooth presented by the molars of the pig, 

 in which the crown carries four columns, severally placed 

 at its corners. When a tooth like the one represented in 

 Fig. 4 becomes worn down by use, the enamel coating each 

 of the columns would be cut through, so as to expose a 

 series of more or less trefoil- shaped surfaces of ivory, 

 each surrounded by a ring of the hard enamel. And it 

 will be obvious that a tooth thus constructed of alternations 

 of substances of different degrees of hardness would act 

 as an efficient millstone. Elephants do not appear, 

 however, to have been by any means satisfied with this 

 comparatively simple kind of tooth, for as we pass upwards 

 in the geological scale we find that there has been a gradual 

 increasing complexity in the structure of the molar teeth 

 of these animals, this being due to a graduated increase 

 in the height of their transverse ridges, accompanied 

 by an increase in the number of the ridges themselves. 

 There is, indeed, an almost perfect structural gradation, 

 now known to exist between the mastodon tooth, repre- 

 sented in Fig. 4, to the teeth of true elephants shown in 

 Fig. 6. Both the latter examples are in a somewhat worn 

 condition, but it will be readily seen that the lozenge- 

 shaped surfaces of ivory, surrounded by enamel, in the 

 tooth of the African elephant, correspond to the transverse 

 ridges of the mastodon's tooth. In the true elephants, 

 however, the open valleys between these ridges (which 

 have now assumed the form of tall, thin, and nearly 

 parallel laminae) have been completely filled up by a third 

 constituent of the tooth, known as the cement. The 



