Page:Popular Science Monthly Volume 12.djvu/700

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THE POPULAR SCIENCE MONTHLY.

ton strongly resembled the living tapir, whose ancestry, to this point, seems to coincide with that of the rhinoceros we are considering. Strangely enough, the rhinoceros line, before it becomes distinct, separates into two branches. In the upper part of the Dinoceras beds we have the genus Colonoceras, which is really a Hyrachyus with a transverse pair of very rudimentary horn-cores on the nasal bones. In the lower Miocene west of the Rocky Mountains this line seems to pass on through the genus Diceratherium, and in the higher Miocene this genus is well represented. Some of the species nearly equaled in size the existing rhinoceros, which Diceratherium strongly resembled. The main difference between them is a most interesting one. The rudimentary horn-cores on the nasals, seen in Colonoceras, are in Diceratherium developed into strong bony supports for horns, which were placed transversely, as in the ruminants, and not on the median line, as in all existing forms of rhinoceros. In the Pliocene of the Pacific coast, a large rhinoceros has been discovered, which may be a descendant of Diceratherium; but, as the nasal bones have not been found, we must wait for further evidence on this point. Returning now to the other branch of the rhinoceros group, which left their remains mainly east of the Rocky Mountains, we find that all the known forms are hornless. The upper Eocene genus, Amynodon, is the oldest known rhinoceros, and by far the most generalized of the family. The premolars are all unlike the molars; the four canines are of large size, but the inner incisor in each jaw is lost in the fully adult animal. The nasals were without horns. There were four toes in front, and three behind. The genus Hyracodon, of the Miocene, which is essentially a rhinoceros, has a full set of incisor and canine teeth; and the molars are so nearly like those of its predecessor, Hyrachyus, that no one will question the transformation of the older into the newer type. Hyracodon, however, appears to be off the true line, for it has but three toes in front. In the higher Miocene beds, and possibly with Hyracodon, occurs a larger rhinoceros, which has been referred to the genus Aceratherium. This form has lost the canine and one incisor above, and two incisors below. In the Pliocene are several species closely related, and of large size. Above the Pliocene in America, no vestiges of the rhinoceros have been found; and our American forms, doubtless, became extinct at the close of this period.

The tapir is clearly an old American type; and we have seen that, in the Eocene, the genera Helaletes and Hyrachyus were so strongly tapiroid in their principal characters that the main line of descent probably passed through them. It is remarkable that the Miocene of the West, so greatly developed as it is on both sides of the Rocky Mountains, should have yielded but a few fragments of tapiroid mammals; and the same is true of the Pliocene of that region. In the Miocene of the Atlantic coast, too, only a few imperfect specimens have been found. These forms all apparently belong to the genus