Page:Popular Science Monthly Volume 12.djvu/692

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T remains now to consider the highest group of the animal kingdom, the class Mammalia, which includes man. Of the existence of this class before the Trias we have no evidence, either in this country or in the Old World, and it is a significant fact that, at essentially the same horizon in each hemisphere, similar low forms of mammals make their appearance. Although only a few incomplete specimens have been discovered, they are characteristic and well preserved, and all are apparently Marsupials, the lowest mammalian group which we know in this country, living or fossil. The American Triassic mammals are known at present only from two small lower jaws, on which is based the genus Dromatherhium, supposed to be related to the insect-eating Myrmecobius, now living in Australia.

Although the Jura of Europe has yielded other similar mammals, we have as yet none of this class from that formation; while, from rocks of Cretaceous age, no mammals are known in any part of the world. This is especially to be regretted, as it is evidently to the Cretaceous that we must look for the first representatives of many of our present groups of mammals, as well as for indications of their more ancient lineage. That some discovery of this nature from the Cretaceous is near at hand, I cannot doubt, when I consider what the last few years have brought to light in the Eocene.

In the lowest Tertiary beds of this country a rich mammalian fauna suddenly makes its appearance, and, from that time through the age of mammals to the present, America has been constantly occupied by this type of life in the greatest diversity of form. Fortunately, a nearly continuous record of this life, as preserved, is now accessible to us, and insures great additions to our knowledge of the genealogy of mammals, and perhaps the solution of more profound problems. Before proceeding to discuss in detail American fossil Mammalia, it is important to define the divisions of time indicated in our Tertiary and Post-Tertiary deposits, as these in many cases mark successive stages in the development of the mammals.

The boundary-line between the Cretaceous and Tertiary in the region of the Rocky Mountains has been much in dispute during the last few years, mainly in consequence of the uncertain geological bearings of the fossil plants found near this horizon. The accompanying