In the next stage represented by such forms as Tetrabelodon longirostris, the elephants gave up the attempt to reach the ground and came to depend on their trunks both for eating and drinking. As the elongated lower jaw is thus no longer necessary, and must have interfered with the free use of the trunk, its anterior exten sion becomes very rapidly reduced in size so that it no longer ex tends in front of the bony upper jaw. In these animals only one milk molar is replaced by a pre-molar and the first two molar teeth have each four or five transverse ridges. The lower tusks are short and rounded whilst the upper tusks become still larger and out wardly directed. In these forms the shortening of the lower jaw allowed the trunk to fall down vertically, as it does in modern ele phants and in external appearance they must have been entirely elephantine.
The later mastodons pass gradually into the elephants by a still further reduction of the lower jaw, the lower tusks becoming quite small and eventually disappearing altogether, finally the front of the lower jaw becomes reduced to a very small down-turned spout which is retained in the living elephants. In the later forms the milk teeth are no longer replaced vertically by pre-molars, whilst such teeth when they do occur must have been pushed out almost immediately by the forward movement of the molar teeth. At the beginning of the series, the second molar tooth in either jaw has four or five ridges, separated from one another by deep valleys in which there is no cement. In an intermediate stage, Stegodon, the
number of ridges varies from 6 to 12 in different forms and the valleys between these ridges become filled up with cement. In the true elephants the ridges are not only more numerous but much higher and the cement forms a plate lying between them. Contin uation of this process leads to the most highly specialized of all elephants, the mammoth, in which the number of ridges in the second molar may be as high as 16, and the whole tooth was extraordinarily deep.
The history of the evolution of the elephant set forth above is ! a mere outline. It is really greatly complicated by the existence of a large number of side branches, many of which migrated into North and some even into South America. Of these side branches much the most striking is that represented by Dinotherium, an animal with an elephant-like body, apparently with exceptionally long limbs and a trunk. This animal is remarkable because its molar teeth throughout the whole history of the genus have only two transverse ridges and are very low crowned. The upper tusks are completely absent whilst the large lower tusks are directed downwards at right angles to the lower jaw. This animal is prob ably of African origin and is found only in that continent and in Europe and India. (See ELEPHANT; MAMMOTH; MASTODON.)