There is usually, or at least often, another point of difference between elephants and mastodons, for many of the latter not only had tusks in the upper, but in the lower jaw, and these are never found in any of the true elephants. The lower tusks are longer and larger in the earlier species of mastodon than in those of more recent age and in the latest species, the common American mastodon, the little lower tusks were usually shed early in life. These afford some hints of the relationships of the mastodon; for in Europe are found remains of a huge beast well called Dinotherium, or terrible animal, which possessed lower tusks only, and these, instead of sticking out from the jaw are bent directly downwards. No perfect skull of this creature has yet been found, but it is believed to have had a short trunk. For a long time nothing but the skull was known, and some naturalists thought the animal to have been a gigantic manatee, or sea cow, and that the tusks were used for tearing food from the bottom of rivers and for anchoring the animal to the bank, just as the walrus uses his tusks for digging clams and climbing out upon the ice. In the first restorations of Dinotherium it is represented lying amidst reeds, the feet concealed from view, the head alone visible, but now it is pictured as standing erect, for the discovery of massive leg-bones has definitely settled the question as to whether it did or did not have limbs. There is another hint of relationship in the upper tusks of the earlier mastodons, and this is the presence of a band of enamel running down each tusk. In all gnawing animals the front, cutting teeth are formed of soft dentine, or ivory, faced with a plate of enamel, just as the blade of a chisel or plane is formed of a plate of tempered steel backed with soft iron; the object of this being the same in both tooth and chisel, to keep the edge sharp by wearing away the softer material. In the case of the chisel this is done by a man with a grindstone, but with the tooth it is performed automatically and more pleasantly by the gnawing of food. In the mastodon and elephant the tusks, which are the representatives of the cutting teeth of rodents, are wide apart, and of course do not gnaw anything, but the presence of these enamel bands hints at a time when they and their owner were smaller and differently shaped, and the teeth were used for cutting. Thus, great though the disparity of size may be, there is a suggestion that through the mastodon the elephant is distantly related to the mouse, and that, could we trace their respective pedigrees far enough, we might find a common ancestor.