622 MASTODON (from /AOOTOS, "nipple," and " tooth "), a name, suggested by the conical or papillary form of the projections on the molar teeth of some of the species, given by Cuvier to a genus of extinct elephant-like animals. Their position in the suborder Proboscidea of the great order Ungulata has been indicated in the article MAMMALIA (p. 425 of the present volume). In size, general form, and principal osteological characters the Mastodons resembled the Elephants. It is by the teeth alone that the two groups ace to be distinguished, and, as shown in the article just referred to, so numerous are the modifications of these organs in each, and so insensibly do they pass by a series of gradations into one another, that the distinction between the two is an arbitrary and artificial one, though convenient and even necessary for descriptive purposes. As in other Proboscideans, the teeth of Mastodons consist only of incisors and molars. The incisors or tusks are never more than a single pair in each jaw. In the upper jaw they are always present and of large size, but apparently never so much curved as in some species of Elephant, and they often have longitudinal bands of enamel, more or less spirally disposed upon their surface, which are not met with in Elephants. Lower incisors, never found in true Elephants, are present throughout life in some species of Mastodon, which have the symphysis of the lower jaw greatly elongated to support them (as in M. angustidens, M. pentelici, and M. longirostris (see fig. 1, C). In the common American species (M. ohioticus, Blumenbach) there were two tusks in the lower jaw in the young of both sexes ; these were soon shed in the female, but one of them was retained in the male. In other species no inferior tusks have been found, at all events in adult life (see figure of M. turicensls). The molar teeth are six in number on each side, increas ing in size from before backwards, and, as in the Elephants, with a horizontal succession, the anterior teeth being lost before the full development of the posterior teeth, which gradually move forward, taking the place of those that have been destroyed by wear. This process is, however, not so complete as in the true Elephants, and as many as three teeth may be in place in each jaw at one time. There is, moreover, in many species a true vertical succession, affecting either the third, or the third and second, or (in one American species, M. produdv.s) the first, second, and third of the six molariform teeth. These three FIG. 1. Mastodon turicensis (Pliocene). From Sismonda. A, B, M. ohioticus ; C, M. longirostris. are therefore reckoned as milk molars, and their successors as premolars, while the last three, which are never changed, correspond to the true molars of those animals in .which the typical dentition is fully developed. The study of the mode of succession of the teeth in the different species of Mastodons is particularly interesting, as it exhibits so many stages of the process by which the very anomalous dentition of the modern Elephants may have been derived by gradual modification from the typical heterodont and diphyodont dentition of the ordinary Mammal. It also shows that the anterior molars of Elephants do not corre spond to the premolars of other Ungulates, but to the milk molars, the early loss of which in consequence of the peculiar process of horizontal forward-moving succession does not require, or allow time for, their replacement by premolars. It must be noted, however, that, in the Mastodon in some resp.octs the least specialized in tooth-structure, the M. ohioticus of North America, no vertical succession of the molars has yet been observed, although vast numbers of specimens have been examined. The Mastodons have, generally speaking, fewer ridges on their molar teeth than the Elephants ; the ridges are also less elevated, wider apart, have a thicker enamel covering, and scarcely any cementum filling up the space between them. Sometimes (as in M. ohioticus) the ridges are simple transverse wedge-shaped elevations, with straight or FIG. 2. Upper Molar of Mastodon arrernensis. From Owen, concave edges. In other species the summits of the ridges are more or less subdivided into conical cusps, and may have accessory cusps clustering around them (as in M. arvernensis, see fig. 2). When the apices of these are worn
by mastication, their surfaces resemble circles of dentine,