The evolution of the Proboscidea was, in a certain sense, very similar to that of the †oreodont family (p. 381) among the Artiodactyla, in that the developmental changes affected chiefly the dentition and the skull, the skeleton of the body and limbs having very early acquired a character which was afterward but little modified. Were the skull and teeth of the lower Miocene _†Gomphotherium_ not known, we should hardly hesitate to refer the skeleton to the genus _Elephas_, and even in the Oligocene _†Palæomastodon_ all the bones of the skeleton, other than the skull, were characteristically and unmistakably proboscidean. On the other hand, the transformations of the teeth and skull were very profound and far-reaching, very much more so than those which took place in the †oreodonts. [Illustration: FIG. 229.—Evolution of the Proboscidea: on the right, a series of skulls; on the left, last lower molar. (After Lull, modified by Sinclair.) N.B. _†Tetrabelodon_ should read _†Gomphotherium_.] In the dentition we may consider separately the development of the tusks and of the grinding teeth. The first step in the known series, as exemplified by _†Mœritherium_, was the enlargement of the second incisor in each jaw to form a tusk which, though actually quite long, was very small when judged by the proboscidean standard. The upper tusk was directed vertically downward and the lower one was procumbent, pointing almost directly forward; the third incisor and the canine were small and in the lower jaw already lost. In the next known stage, _†Palæomastodon_, all of the anterior teeth, except the tusks, had been suppressed; the upper tusks were longer and more curved and of an oval cross-section; they extended less directly downward and more forward, while the enamel was restricted to the outer side of the tusk; the lower tusks were more fully procumbent than in the preceding genus. The third stage, that of the lower Miocene _†Gomphotherium_, showed the upper tusks greatly elongated and directed more forward than downward, while the lower tusks were but little larger than before. From the middle Miocene two phyla may be distinguished by the tusks alone; in one, which was not destined to long life, the lower pair increased greatly both in length and in diameter, while in the other series they rapidly diminished and eventually disappeared. Even in the Pleistocene, however, the American †Mastodon had remnants of these tusks in the males. In the later †mastodons, the †stegodonts, and true elephants, the upper tusks, which alone remained, lost the enamel bands and attained enormous proportions, differing in the various genera and species in the extent and direction of curvature. An aberrant mode of tusk development was to be seen in the †dinotheres, in which the upper pair was suppressed and the lower pair enlarged and so curved that the points were directed backward.