MASTODON (from p,ao-rOs, "nipple," and 6801.1s., " tooth "), a name, suggested by the conical or papillary form of the projections on the molar teeth of some of the species, given by Olivier to a genus of extinct elephant-like animals. Their position in the suborder Proboscidea of the great order Ung viola 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 are 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, pentelici, and M. longirostris (see fig. 1, C). In the common American species (M. okioticns, 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. turicensis).
The molar teeth are six in number on each side, increasing 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. productus) the first, second, and third of the six molariform teeth. These three MASTODON 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 correspond 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 respects the least specialized in tooth-structure, the 11I. oltioticus 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 251 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 .11f. arvernensis, see fig. 2). When the apices of these are worn by mastication, their surfaces resemble circles of dentine, characteristic of some species.
Certain of the molar teeth of the middle of the series in both Elephants and Mastodons have the same number of principal ridges, and those in front of them have fewer and those behind a greater number. These teeth were distinguished as " intermediate" molars by Dr Falconer, to whose extensive and conscientious researches we owe much molars (or the third, fourth, and fifth of the whole series). In the Elephants the number of ridges on the intermediate molars always exceeds five, but in the Mastodons it is nearly always three or four, and the tooth in front has usually one fewer and that behind one more, so that the ridge formula (i.e., a formula expressing the number of ridges on each of the six molar teeth) of most Mastodons can be reduced either to 1, 2, 3, 3, 3, 4 or 2, 3, 4, 4, 4, 5. The former characterizes the section called Trilophodon, and the latter that called Tetralophodon by Dr Falconer. These divisions are very useful, as under one or the other all the present known species of Mastodon can be ranged, means so constant as implied by the mathematical form ulm given above. Their exact enumeration is even difficult in many cases, as " talons " or small accessory ridges at the hinder end of the teeth occur in various stages of development, until they take on the character of true ridges. Transitional conditions have also been shown, at least on towards the condition of dental structure characteristic of the true Elephants.
The range of the genus Mastodon in time was from the middle of the Miocene period to the end of the Pliocene in the Old World, when they became extinct ; but in America several species - especially the best-known, owing to the abundance of its remains, which has been variously called M. ohioticus, M. americanus, and Al. giganteus - survived quite to a late Pleistocene period.
The range in space will be best indicated by the following list of the generally recognized species. 1. Trilophodont series - AT. ang7tstidens,borsoni, pentelici, pyrenaicus, tapiroides (or turiccnsis), virgatidens, from Europe ; M. falconeri and pandionis, from India ; Al. ohioticus, obscurus, and productus, North America ; and AL andium. and humboldtii, South America. 2. Tetralophodont series - M. arvernensis, AL dissimilis and longirostris, from Europe ; latidens, sivalensis, and perantensis, from India ; M. mirificus, from North America. The only two of which remains have been found in Great Britain are AL arvernensis and M. borsoni, both from the crags of Norfolk and Suffolk.
The range of the genus was thus very extensive, and it has even been supposed to reach to Australia, where no Ungulate mammal has ever been proved to exist. This supposition until very recently has been based upon the evidence of a single molar tooth of an animal undoubtedly belonging to Mastodon, and alleged to have been brought from near Boree Creek, an affluent of the Lachlan river in the Ashburnharn district, New South Wales, by the late Count Strzelecki, and described by Professor Owen in 1844 under the name of Al. australis. Its identity with the South American AL andium has, however, been shown by Dr Falconer, who has thrown grave doubts upon the locality assigned to the specimen. A fragment of a tusk, of the Australian origin of which there is less question, and which presents the characteristic structure only known at present in Elephants and Mastodons, has been lately described by Professor Owen (Proc. Roy. Soc., March 30, 1882). It was found in a drift-deposit of a ravine in a district of Darling Downs, 60 miles to the eastward of Moreton Bay, Queensland. Unfortunately no other portions of the remains of the animal to which it belonged have been discovered.
Bibliography. - Cuxier, Ossemens FossiTes; Falconer and Cnutley, Fauna Antigua Slvalensis. 18i6-47; II. Falconer, Paleontological Memotirs, edited by C. Murchison, 1868 ; Warren, Description of the Skeleton of Mastodon gigan tens, 1852; Owen, British Fossil Manunats; Larte, "Sur Is dentition des Froboscidiens, be ," in Bull. de, la Soc. Geologigue de France, ser. 2, vol. xvi. p. 469, 1859; A. Gaudry, Anialaux Possiles et Geologic de l'Attigue, 1862-67 ; J. Leidy, Contributions to Extinct Vertebrate Fauna of the Western Territories, vol. 1., 1873; IL Lydekker, "Siwalik and Nalbada Proboseldea," in Memoirs of the Geological Survey of India, 1880. (W. H. F.)