Page:EB1911 - Volume 22.djvu/421

Rh E. bombifrons and E. clifti, typically from the Lower Pliocene of India and Burma, but some of which extend eastwards to Java, Borneo, China and Japan. These constitute the group (or genus) Stegodon, and are characterized by the lowness of the crowns of the molar teeth, in which the tall plates of the more typical elephants are reduced to low ridges with more or less completely open valleys between them; the number of ridges in each tooth is always much lower than in the corresponding teeth of the typical elephants. Premolars, vertically replacing the anterior molars, were often developed. These stegodont elephants appear to have been confined to India and the countries farther east, and exhibit an almost complete transition, so far as dental characters are concerned, to the mastodons of the same region.

Mastodon.—The connexion between the stegodont elephants and the mastodons (see ) is formed by the Indian and Burmese Mastodon latidens and M. cautleyi. In fact the main distinction between these animals and the stegodont elephants is the smaller number of ridges in the third, fourth and fifth molars, which is usually four, and never exceeds five, whereas in the stegodonts it is at least six and the numbers are not the same in each of the three teeth. In the two species named the transverse ridges are more or less continuous. Many other species, such as the European M. arvernensis (see fig. 2 in art. ) and the Indian M. sivalensis, have, however, the ridges broken up into columns, or cones, more or less alternately arranged, and thus blocking the intermediate valleys. In these species, which are of Pliocene age, there are four ridges in molars 3, 4 and 5; but in the Pleistocene North American M. americanus (as well as in many other species) these are reduced to three in each of the aforesaid teeth. The lower jaw of the latter species frequently shows small tusks, which are, however, generally shed in mature age. Premolars, which vertically replace some of the anterior molars (milk-molars), are developed in many species, although not in M. americanus. Species of the genus are found over the greater part of the world, inclusive of Europe, Asia and North and South America; M. humboldti being the best known South American species. A single tooth referable to this or the next genus has been obtained from South Africa.

Tetrabelodon.—The more primitive mastodons constitute the genus Tetrabelodon, and are characterized by the presence of a pair of short chisel-shaped tusks in the lower jaw, which is prolonged into a trough-like chin for their support; tusks being also present in the upper jaw. These animals were provided with a snout-like muzzle instead of a trunk (see ). Their birthplace was Africa; the Miocene European M. angustidens having been discovered in Egypt in strata overlying those from which were obtained the remains of the under-mentioned more primitive genera. Tetrabelodont mastodons were, however, by no means confided to the Miocene, Tetrabelodon langirostris occurring in the Lower Pliocene of Europe, and T. pandionis in that of India. Most of these four-tusked mastodons were smaller animals than modern elephants.

Palaeomastodon.—No proboscidean earlier than Tetrabelodon occurs in Europe, but the group is represented in the Upper Eocene of Egypt by a smaller and more primitive type known as Palaeomastodon. This genus resembles Tetrabelodon in having four pairs of tusks, but differs in the less elephant-like skull, and the simpler character of the molar teeth, of which five pairs were in use at one time, whereas in Tetrabelodon and Mastodon there were never more than two pairs and a portion of a third in simultaneous wear.

Moeritherium.—The earliest representative of the proboscidean stock at present known is Moeritherium, from the Middle Eocene of Egypt, which includes still smaller animals, whose relationship to Elephas would scarcely be realized were it not for the intermediate links. All six pairs of cheek-teeth (pm. 2-m. 3,

fig. 3) were in use at once, and there was a comparatively full series of teeth in the front of the jaws; while the premolars were preceded by milk-molars in the normal manner. Very significant is the enlargement of the second pair of incisors in each jaw, thereby foreshadowing the tusks of Tetrabelodon. There was, however, no lengthening of the chin, so that the muzzle was probably of normal proportions. This animal was not larger than a tapir.

Dinotherium.—The huge proboscidean from the Lower Pliocene and Middle Miocene strata of Europe and India, known as Dinotherium, indicates a type off the line of descent of the elephants. Upper tusks were apparently wanting, but the lower jaws carried a pair of large tusks bent downwards in a peculiar manner (fig. 4). The cheek-teeth formed five pairs, all in use at one time, and premolars vertically replacing milk molars in the ordinary fashion. The ridge-formula of the permanent teeth of the cheek series was 2.2.3.2.2.



Barytherium and Pyrotherium.—Very problematical are the