420 



UNGULATA 



of replacement found in the true Elephants. The intermediate 

 conditions occur in the various species of Mastodon. In this genus 

 the enamel-covered transverse ridges of each tooth are generally 

 more numerous than in Dinotheriitm, and often complicated by- 

 notches dividing their edge or by accessory columns attached to 

 them, but in the unworn tooth they stand out freely on the surface 

 of the crown, with deep valleys between (Fig. 179, I). In the 

 Elephants the ridges are still further increased in number, and con- 

 sequently narrower from before backwards, and are greatly extended 



M 



I\r 



Fig. 179.— Longitudinal sections of the crown of a molar tooth of various Proboscideans, 

 showing stages in the gradual modification from the simple to the complex form. I, Mastodon 

 americanus; II, Elephas insignis; III, Elephas africanus; IV, Elephas primigenius. The dentine 

 is indicated by transverse lines, the cement by a dotted surface, and the enamel is black. 



in vertical height, so that, in order to give solidity to what would 

 otherwise be a laminated or pectinated tooth, it becomes necessary 

 to envelop and unite the whole in a large mass of cement, which 

 completely fills up the valleys, and gives a general smooth appear- 

 ance to the organ when unworn ; but as the wear consequent upon 

 the masticating process proceeds, the alternate layers of tissue 

 of different hardness — cement, dentine, and enamel — which are 

 disclosed upon the surface form a fine and very efficient triturating 

 instrument. The modification of the tooth of a Mastodon into that 

 of an Elephant is therefore precisely the same in principle as that 

 of the molar of a Palseotherium into that of a Horse, or of the 



