The proboscidians.  This unique order of hoofed mammals, of which the elephant is the sole survivor, began, so far as known, in the Eocene, in Egypt, with a piglike ancestor the size of a small horse, with cheek teeth like the Mastodon’s, but wanting both trunk and tusks.  A proboscidian came next with four short tusks, and in the Miocene there followed a Mastodon (Fig. 346) armed with two pairs of long, straight tusks on which rested a flexible proboscis.

The dinothere was a curious offshoot of the line, which developed in the Miocene in Europe.  In this immense proboscidian, whose skull was three feet long, the upper pair of tusks had disappeared, and those of the lower jaw were bent down with a backward curve in walrus fashion.

In the true elephants, which do not appear until near the close of the Tertiary, the lower jaw loses its tusks and the grinding teeth become exceedingly complex in structure.  The grinding teeth of the mastodon had long roots and low crowns crossed by four or five peaked enameled ridges.  In the teeth of the true elephants the crown has become deep, and the ridges of enamel have changed to numerous upright, platelike folds, their interspaces filled with cement.  The two genera—­Mastodon and Elephant—­are connected by species whose teeth are intermediate in pattern.  The proboscidians culminated in the Pliocene, when some of the giant elephants reached a height of fourteen feet.

The artiodactyls comprise the hoofed Mammalia which have an even number of toes, such as cattle, sheep, and swine.  Like the perissodactyls, they are descended from the primitive five-toed plantigrade mammals of the lowest Eocene.  In their evolution, digit number one was first dropped, and the middle pair became larger and more massive, while the side digits, numbers two and five, became shorter, weaker, and less serviceable.  The four-toed artiodactyls culminated in the Tertiary; at present they are represented only by the hippopotamus and the hog.  Along the main line of the evolution of the artiodactyls the side toes, digits two and five, disappeared, leaving as proof that they once existed the corresponding bones of palm and sole as splints.  The two-toed artiodactyls, such as the camels, deer, cattle, and sheep, are now the leading types of the herbivores.

Swine and Peccaries are two branches of a common stock, the first developing in the Old World and the second in the New.  In the Miocene a noticeable offshoot of the line was a gigantic piglike brute, a root eater, with a skull a yard in length, whose remains are now found in Colorado and South Dakota.