420 UNGULATA 
of replacement found in the true Elephants. The intermediate 
conditions occur in the various species of J/ustoden. In this genus 
the enamel-covered transverse ridges of each tooth are generally 
more numerous than in Dinotherium, 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 
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Fia. 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 trituratine 
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 Palzotherium into that of a Horse, or of the 
