VII.The division of various mastodons into proper groups is no mean task. The palaeontologist must play the part of prehistoric animal trainer and only a skilled hand can keep the animals subdued and each where he belongs. Until recently, literature on the subject was hopelessly confused. The early long-jawed Mastodons were called Tetrabelodons or Four-tuskers and the South American forms akin to them were termed Dibelodons or Two-tuskers. As regards molar teeth, those with three transverse ridges branded their wearers as Trilophodons and ones with four-ridged teeth, as Tetralophodons. Long-jawed animals were Longirostrines and short-jawed, Brevirostrines. Enamel-banded tusks also came in for their share of attention. As if this were not enough to confuse the average mind, molar tooth crowns on closer inspection exhibited not only single but double trefoils, also accessory cones so persistently displayed that they too had to be considered.
The Long-Jawed Tetralophodon
Reverting to the early name Tetrabelodon, we find that some of them have three-ridged teeth while others, later comers, have four, also short as well as long jaws. Evidently Tetrabelodon as a title is only partly descriptive. The same is true of most names, very few of which, in the light of recent discoveries, are free from exceptions. The grouping of the various mastodons is now in process of preparation, and with continued discoveries, time will bring it to proper completion.
The truth of the matter is that the scientific world has but recently recognized that primitive elephant genealogy is not a matter of a few well-defined and successive groups, but a tangled mass of ramifications which had to do with trunk, teeth, tusks and jaws. And yet these ramifications bore no evidence of confusion, for in each type the characteristics are too persistent to be considered as haphazard. All was done for a purpose whose aim was to give each beast every advantage according to his attainments. With a fixed standard of living, the tendency of parts would have been to assume comparatively few forms. But it must be remembered that even the Oligocene Proboscidians tended to diverge, and as we know, the Mastodon was a born traveller. In his ramblings he encountered innumerable favorable and unfavorable conditions of climate, topography and the proximity of other animals, all of which bore directly upon his food supply and health in general. Here his inborn power of adaptation was turned in various ways to make the best of every situation in which he found himself, and the most successful of these adaptations persisted as alterations in tooth, jaw and head structure. Mastodon history is a matter of several million years at least, and in that great length of time it is not surprising that many changes were accomplished. And yet there is little or no indisputable evidence that one group developed into another; rather, a replacing of old forms with new ones with the two living for a time, side by side. So it was with the Long-jawed Trilophodon, even to the short-jawed stage and in their later years at least, both existed contemporaneously with another well-defined group, the Long-jawed Tetralophodon.
Were it simply a matter of long jaws and four tusks, this mastodon would be included with the Trilophodon. The two are very closely related, but there is one very important distinction. The Tetralophodon had four cross-ridges on his second molars instead of three. These ridges were complicated additionally by accessory cones, making double trefoils instead of single. The tusks were long and powerful and but slightly curved. All of them were equipped with enamel strips or bands to resist abrasion. This race is first recorded in the early Pliocene of India (Tetralophodon Corrugatus) and then in Central Europe (Tetralophodon Longirostris) where remains of the beast are often met with. The North American representative (Tetralophodon Campester) hails from the Pliocene of Kansas. The increase in number of cross-crests from three to four is a notably progressive feature as will be demonstrated later. The addition of trefoils however cannot be construed as a mark of advancement, rather the contrary, as it tended to produce a pig-like tooth of no distinctive pattern but a promiscuous assemblage of knobs, large and small.
The last molar of a Nebraskan species (Tetralophodon Barbouri) differs from The American type (Tetralophodon Campester) in having the crest and valley pattern obscured, by not merely one, but two clover-leafed or trefoiled hillocks, thereby inclining to swinish construction. The third and last molar tooth, which in all mastodons except the Beak Jawed Ryncotherium, is much the largest and with the most cross-crests, has in Tetralophodon Barbouri seven and a half crests, one more than in Tetralophodon Campester. The molar crowns of the former are covered with a third and hitherto unknown tooth substance known as cement; this in addition to the tooth body of ivory or dentine and the wearing-surface of enamel. Apparently Tetralophodon Barbouri is an advanced type, but as our knowledge of him is based only on a single molar tooth, his family group awaits further definition, as is true with many others whose remains are too few and fragmentary to permit of precise determination.
The Long-jawed Tetralophodon became extinct before the close of the Pliocene period and was the last of his kind, for the Mastodon world in general had begun to progress along new lines. Jaw development had been overdone, in that it interfered too much with the rapidly growing trunk. Intuition must have taught the value of the latter instrument and all Mastodons were by this time disposed to give it every possible advantage. Like in Debelodon, lower jaws shortened up and their tusks became obsolete. The upper tusks lost their enamel bands and tended to curve upward. In this manner the old long-jawed race finally yielded its place to a new order the "Pig-tooth," otherwise known as Brevirostris, the Short-jawed Mastodon.
[from the original, typewritten draft with hand-written corrections - GL,III, ed.]