FOSSIL MAMMALS IN NORTH AMERICA
DR. LEIDY’S new work on thc extinct mammalia and fauna of Dakota and Nebraska, to which is appended a Synopsis of the Mammalian Remains of North
Amcrica, fills a wide gap in Palaxontology. It occupies
the whole of the seventh volume of the Journal of the
Academy of Natural Sciences of Philadelphia, and is ac-
companicd by a preface on the geology of the Tertiary
Strata of Dakota and Nebraska, by Dr. Hayden. Alto-
gether it forms the most important contribution to our
knowledge of fossil mammals which has been made since
Prof. Gaudry published his famous “ Animaux Fossiles
ct Geologie de I'Attique.” These two books, indeed,
stand in closc relation to one another, for in the one the chief interest centres in the Miocene fauna, which is the subject matter of the other. I propose to give an outline of Dr. Leidy’s work, and to show the relation which the
American Mammalia bore to those of Europe, from
the Miocene down to the “ Quaternary,” or Post-glacial
epoch.
At the close of the Cretaccous period, writes Prof. Hayden, the ocean which had “rolled uninterruptedly across the area now occupied by the Rocky Mountains” began to grow shallow, until at last a long barrier of land gradually rose above the waves, and separated the Atlantic from the Pacific. This elevatory movement culminated in the Rocky Mountain range in the United States, and probably has been going on from the Cretaceous period down to the present day. In the early Tertiary epoch enormous lakes occupied the basin of the Mississippi. The “ great lignite basin,” for instance, extends far south- ward, possibly even to California, westward far over the mountains to Utah, and possibly to the Pacific, and north- ward probably to the Arctic Sea, interrupted here and there by the upheaval of mountain ranges.” The strata which testify to the former existence of this great lake, consist of layers of clay and sand, and numerous beds of lignite, varying in thickness from a few inches to twelve or fifteen feet. In its lower portion an oyster is the characteristic fossil, which by its stunted growth implies a change from salt to brackishwater, while in the rest of the formation there are freshwater shells of the genera Melania and Corbicula. “ The occurrence of immense fan palms, and many other plants now growing only in tropical climates, points directly to the conclusion that along the shores of this great lake there grew most luxuriant forests, equalled only by those now existing in Central America or Brazil.” The date of this lignite formation is possibly Eocene, and certainly pre-Miocene. Some of these lakes continued to exist as late as the Pliocene epoch.
The * White River group” of rocks consists of white indurated clays, sandstones, and conglomerate marls and sands, upwards of a thousand feet thick, which occupy an area of at least 100,000 square miles on the Eastern flank of the Rocky Mountains. It is purely a fresh-water formation. In Nebraska it is eroded by pluvial and torrential action into quaint pinnacles and
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fantastic shapes of every sort, and into deep barren ravines that recall to mind Dr. Falconer’s description of some parts of the Sevalik Hills. From the difficulty of traversing the 5,000 square miles which are cut up in this way the district is known to the Canadian voyagers as ‘“ Mauvaises Terres,” and to the Indian hunters as the “Bad Grounds.” It has furnished the larger portion of the Mammalia described by Dr. Leidy, and is unequivocally of Miocene Age. The Loup River Strata resting on the White River group, consist of sand and a few layers of limestone, and contain the remains of land and fresh water Testacea and Mammalia. In Nebraska, the sand is so incoherent that it forms a series of ever shifting dunes, which occupy an area of not less than 20,000 square miles. The remains of Elephant and Mastodon and others show that it belongs to the Pliocene, irrespective of the stratigraphical evidence. The Post-tertiary deposits are represented in the same region by a yellow siliceous marl, most fully developed along the Missouri River, and in the valley of the Plate. It is from three to five hundred feet in thick- ness, and contains well-known Post-glacial fossils.
In treating of the Mammalia yielded by these different formations, Dr. Leidy has followed the example of Prof. Agassiz in the needless multiplication of species. Natu- ralists fall into two classes, according to the ideas which they bring to bear on their work: the one fix their attention on the variability and points of resemblance manifested by suites of organic remains, and stretch the name of species as far asit will go ; the other give specific value to minute differences of size and form, which, in a larger series of specimens, either recent or fossil, would be found value- less in classification. In this work Dr. Leidy has joined himself to the latter class, and has marked every small variation by a specific, and, in some cases, even a generic name, and by so doing has added, without any necessity, to the heavy burden of synonyms which scientific litera- ture has to put up with. He believes that the American is a ‘“ peculiar fauna,” and, that even if no difference be- tween European and American fossils can be detected, their geographical separation is evidence that they be- long to different species. For example, in the description of a new species, Lguus excelsus, he admits that ““ it is not improbable that part of the specimens looked upon as fossils may be the remains of the Mustang, or recent wild horse of our western wilderness.” Nevertheless, he holds to his own undefined specific name. This fault is carried to an extreme, in the case of the extinct family Anchi- theriidee. On the small foundation of one tooth, which, ‘“‘in general proportions and construction,” and “in size,” is “nearly the same as the teeth of Anchitherium Aurelianense,” a new genus, Anchippus, is based; while four milk molars, which “ in form, mode of insertion, and general constitution and size, bear a near resemblance to those of the same species,” constitute the basis of the second new genus Parahippus. This mode of treat- ment runs more or less throughout the work, and renders it of less value than might have been expected from the importance of the subject-matter. But, nevertheless, it is a mine of information to which Tertiary naturalists will resort for many long years to come.
I will now pass on to the consideration of the leading features of the Miocene fauna., On the borders of the ancient Miocene lake, from which the Mauvaises Terres
