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ON THE EXTINCT ANIMALS OF NORTH AMERICA.

BY PROFESSOR WILLIAM HENRY FLOWER, F.R.S.

[PLATE CXXXVIII.]

FEW ring the last few years than that of the past history of

NEW branches of knowledge have received greater accessions

the living creatures which have peopled the earth.

I propose in the present instance to call attention to some of the results that have been achieved, mostly within the last four or five years, by a small but energetic band of explorers upon a limited part of the earth's surface; results the greatness of which already is only equalled by the promise they give of future still more important extensions of knowledge.

It is mainly through the agency of the admirably conducted geological and geographical surveys of the Western Territories, made by the United States Government, under the direction of Dr. F. V. Hayden, that the subjects to which I shall refer have been brought to light; surveys which are giving to the world, in an excellent series of publications, rich funds of information upon the physical geography, mineralogy, geology, paleontology, zoology, and botany of that hitherto little known but most remarkable region of the earth embracing and bordering the great range of the Rocky Mountains. For the special knowledge which we in England possess of the vertebrate fossils which have been discovered by these surveys, we are greatly indebted to the excellent descriptions of Professor Joseph Leidy of Philadelphia, who in two large and beautifully illustrated volumes* has given the results of his investigations upon them. More recently two other naturalists, Professor E. D. Cope of

* "Extinct Mammalian Fauna of Dakota and Nebraska, with a Synopsis of the Mammalian Remains of North America," Journ. Acad. Nat. Science,' Philadelphia, 1869; and "Contributions to the Extinct Vertebrate Fauna of the Western Territories," 'Report of the U. S. Geological Survey of the Territories,' Washington, 1873.

Philadelphia, and Professor O. C. Marsh of Yale College, have taken the subject in hand, both as explorers and describers.*

It must be premised that the material has come to hand so rapidly during the last three or four years that most of the information which has hitherto been given to the world, especially by the two last-named palæontologists, is in a very provisional and fragmentary state; and that until the flood of new discovery begins to ebb, and the few labourers in this plentiful harvest-field have leisure to prepare careful, elaborate, and, above all, well-illustrated descriptions of the specimens, we shall remain in much uncertainty about the real nature and relations of many of the animals of that strange old fauna, which at present are to us little more than names.

I must presume that readers are familiar with the main epochs of time into which geologists have divided the earth's history. For the present purpose we shall only have to refer to the latest of these, the Tertiary, representing how many millions of years we cannot say; and which, for convenience, is generally subdivided into four sub-epochs, the Eocene, the Miocene, the Pliocene, and the Pleistocene, the end of which brings us to the period in which we now dwell. Of course it is not implied by this division that there was any sudden break or interruption of the steady course of the world's progress between these periods. They are merely artificial and arbitrary, though convenient stages, and pass insensibly one into the other; but without the use of some such terms we could not fix the epoch of any particular event or set of events. In geology we know nothing of centuries. We have no kings' reigns, as in political history, to mark the course of time, so we speak of "Miocene" much in the same vague kind of sense in which we speak of the "Middle Ages" in our chronology of the historical events in Europe.

The first evidence of mammalian remains in strata of Miocene age in Western America was that made known in 1846 by Dr. Hiram A. Prout, of teeth then supposed to belong to a gigantic species of Palæotherium, and subsequently described by Leidy under the name of Titanotherium. This was the commencement of that interesting series of discoveries, which have now made the "Mauvaises Terres," or "Bad Lands," of the White River of Dacota, classical ground to the palaeontologist. But it was not until 1869 that the older beds on the western side of the Rocky Mountains were explored, and the more ancient Eocene land fauna of North America brought to light. In that

I am glad to take the opportunity of thanking Professors Hayden, Leidy, Marsh, and Cope, for their kindness in sending me copies of all their numerous memoirs bearing upon the subject of this article.

year commenced the explorations in the vicinity of Fort Bridger, a military post situated in the south-west corner of Wyoming Territory, which have yielded such an abundant harvest, and the locality of which is thus graphically described by Professor Leidy:

"Fort Bridger occupies a situation in the midst of a wide plain, at the base of the Uintah Mountains, and at an altitude of nearly seven thousand feet above the ocean level. The neighbouring country, extending from the Uintah and Wahsatch Mountains on the south and west to the Wind River Range on the north-east, at the close of the Cretaceous epoch, appears to have been occupied by a vast fresh-water lake. Abundance of evidence is found to prove that the region was then inhabited by animals as numerous and varied as those of any other fauna, recent or extinct, in other parts of the world. Then, too, a rich tropical vegetation covered the country, in strange contrast to its present almost lifeless and desert condition.

"The country appears to have undergone slow and gradual elevation; and the great Uintah Lake, as we may designate it, was emptied, apparently in successive portions, and after long intervals, until finally it was drained to the bottom. The ancient lake-deposits now form the basis of the country, and appear as extensive plains, which have been subjected to a great amount of erosion, resulting in the production of deep valleys and wide basins, traversed by Green River and its tributaries, which have their sources in the mountain boundaries. From the valley of the Green River the flat-topped hills rise in succession, as a series of broad table-lands or terraces, extending to the flanks of the surrounding mountains.

"The fossils which form the subjects of our communication for the most part were derived from the more superficial deposits of the great Uintah basin, which Professor Hayden has distinguished as the Bridger group of beds. These compose the terraces or table-lands in the neighbourhood of Fort Bridger, and consist of nearly horizontal strata of variously-coloured indurated clays and sandstones. As the beds wear, through atmospheric agencies, on the naked declivities of the flattopped hills, the fossils become exposed to view, and tumble down to the base of the hills among the crumbling débris of the beds."

The immense length of time that this ancient lake has existed may be inferred from the fact that the mud or sand deposited in it has accumulated to more than a mile of vertical thickness.

It is from this and from neighbouring localities systematically explored only during the last four or five years, both by the Government surveys and by expeditions organised for the purpose from Yale College, that most of the remarkable animals