Another elephantine monster, evidently allied to the Mammoth, was the Mastodon, a creature which there is reason to think was contemporary, in America, with the men of prehistoric age. It was so named by Baron Cuvier to distinguish it from the Mammoth, with which it was by others considered identical; and his discrimination of the two forms marked an important and early step in the history of palæontology. The chief difference between these two extinct types lies in their molar teeth. These, on cutting the gum, must have exhibited a number of somewhat conical protuberances of a mammiform appearance; hence the name.[71] As these points were worn down by mastication, the surface of the tooth showed a series of discs of various sizes. The teeth were covered by a very thick coat of dense, brittle enamel. There are, however, differences in the bony framework of the animal, as well as in its general proportions, which serve to distinguish it from the Mammoth; but it will not be necessary to enter into these matters here, for this is difficult ground, even to the student who is well versed in anatomy. Notwithstanding a vast amount of observation on the subject, considerable differences of opinion have prevailed among palæontologists with regard to the proper relation of the Mastodon to the Mammoth and living elephants. [71] Greek--_mastos_, teat; _odous_, _odontos_, tooth. [Illustration: Fig. 54.--Skeleton _Mastodon arvernensis_, Pliocene, Europe.] At the entrance of the Geological Gallery in the Natural History Museum, South Kensington, the reader will see a magnificent skeleton of an American Mastodon, of which more presently. On this specimen our artist has based his restoration, Plate XXI. A large part of the great gallery referred to is devoted to the fossil remains of proboscideans; that is, creatures provided with a long proboscis, or trunk, such as elephants and Mastodons. This collection, from widely different quarters, is the largest and most complete in the world. By comparing the specimens of teeth in the cases, and looking at the fine specimens of skulls, and the numerous bones and tusks in the side cases, the reader will carry away a better idea than we can convey by description. Fig. 54 shows the skeleton of Mastodon arvernensis with two very long tusks. Mastodon augustidens had four tusks, two in each jaw, but one of those in the lower jaw sometimes dropped out as the animal grew older. [Illustration: Plate XXI. THE MASTODON OF OHIO, M. AMERICANUS.]