Rodentia.—These consist mainly of genera and species still living in North America; the only important exceptions being a species of the South American capybara (Hydrochœrus) in South Carolina; and Praotherium, an extinct form of hare, found in a bone cave in Pennsylvania.
Edentata.—Here we meet with a wonderful assemblage, of six species belonging to four extinct genera, mostly of gigantic size. A species of Megatherium, three of Megalonyx, and one of Mylodon—huge terrestrial sloths as large as the rhinoceros or even as the largest elephants—ranged over the Southern States to Pennsylvania, the latter (Mylodon) going as far as the great lakes and Oregon. Another form, Ereptodon, has been found in the Mississippi Valley.
Marsupialia.—The living American genus of opossums, Didelphys, has been found in deposits of this age in South Carolina.
Remarks on the Post-Pliocene fauna of North America.—The assemblage of animals proved, by these remains, to have inhabited North America at a comparatively recent epoch, is most remarkable. In Europe, we found a striking change in the fauna at the same period; but that consisted almost wholly in the presence of animals now inhabiting countries immediately to the north or south. Here we have the appearance of two new assemblages of animals, the one now confined to the Old World—horses, camels, and elephants; the other exclusively of South American type—llamas, tapirs, capybaras, Galera, and gigantic Edentata. The age of the various deposits in which these remains are found is somewhat uncertain, and probably extends over a considerable period of time, inclusive of the Glacial epoch, and perhaps both anterior and subsequent to it. We have here, as in Europe, the presence and apparent co-existence in the same area, of Arctic and Southern forms—the walrus and the manatee—the musk-sheep and the gigantic sloths. Unfortunately, as we shall see, the immediately preceding Pliocene deposits of North America are rather poor in organic remains; yet it can hardly be owing to the imperfection of the record of this period, that not one of the South American types above numerated occurs there, while a considerable number of Old World forms are represented. Neither in the preceding wonderfully rich Miocene or Eocene periods, does any one of these forms occur; or, with the exception of Morotherium, from Pliocene deposits west of the Rocky Mountains, any apparent ancestor of them! We have here unmistakable evidence of an extensive immigration from South into North America, not very long before the beginning of the Glacial epoch. It was an immigration of types altogether new to the country, which spread over all the southern and central portions of it, and established themselves sufficiently to leave abundance of remains in the few detached localities where they have been discovered. How such large yet defenceless animals as tapirs and great terrestrial sloths, could have made their way into a country abounding in large felines equal in size and destructiveness to the lion and the tiger, with numerous wolves and bears of the largest size, is a great mystery. But it is nevertheless certain that they did so; and the fact that no such migration had occurred for countless preceding ages, proves that some great barrier to the entrance of terrestrial mammalia which had previously existed, must for a time have been removed. We must defer further discussion of this subject till we have examined the relations of the existing faunas of North and South America.
Tertiary Period.
When we get to remains of the Tertiary age, especially those of the Miocene and Eocene epochs, we meet with so many interesting and connected types, and such curious relations with living forms in Europe, that it will be clearer to trace the history of each order and family throughout the Tertiary period, instead of considering each of the subdivisions of that period separately.
It will be well however first to note the few American Post-Pliocene or living genera that are found in the Pliocene beds. These consist of several species of Canis, from the size of a fox to that of a large wolf; a Felis as large as a tiger; an Otter (Lutra); several species of Hipparion; a peccary (Dicotyles); a deer (Cervus); several species of Procamelus; a mastodon; an elephant; and a beaver (Castor). It thus appears that out of nearly forty genera found in the Post-Pliocene deposits, only ten are found in the preceding Pliocene period. About twelve additional genera, however, appear there, as we shall see in going over the various orders.
Primates.—Among the vast number of extinct mammalia discovered in the Tertiary deposits of North America, no example of this order had been recognized up to 1872, when the discovery of more perfect remains showed, that a number of small animals of obscure affinities from the Lower Eocene of Wyoming, were really allied to the lemurs and perhaps also to the marmosets, the lowest form of American monkeys, but having a larger number of teeth than either. A number of other remains of small animals from the same formation, previously supposed to be allied to the Ungulata, are now shown to belong to the Primates; so that no less than twelve genera of these animals are recognized by Mr. Marsh, who classes them in two families—Limnotheridæ, comprising the genera Limnotherium, (which had larger canine teeth), Thinolestes, Telmatolestes, Mesacodon, Bathrodon, and Antiacodon of Marsh, with Notharctos, Hipposyus, Microsyops, and Palæacodon previously described by Leidy;—and Lemuravidæ, consisting of the genera Lemuravus (Marsh) and Hyopsodus (Leidy). The animals of the latter family were most allied to existing lemurs, but were a more generalized form, Lemuravus having forty-four teeth, the greatest number known in the order. These numerous forms ranged from the size of a small squirrel to that of a racoon. It is especially interesting to find these peculiar lemuroid forms in America, just when a lemur has been discovered of about the same age in Europe; and as the American forms are said to show an affinity with the South American marmosets, while the European animal is most allied to a West African group, we have evidently not yet got back far enough to find the primeval or ancestral type from which all the Primates sprang.
About the same time, in the succeeding Miocene formation, true monkeys were discovered. Mr. Marsh describes Laopithecus as an animal nearly the size of the largest South American monkeys, and allied both to the Cebidæ and the Eocene Limnotheridæ. Mr. Cope has described Menotherium from the Miocene of Colorado, as a lemuroid animal, the size of a cat, and perhaps allied to Limnotherium. More Miocene remains will, no doubt, be discovered, by which we shall be enabled to trace the origin of some of the existing forms of South American monkeys; and perhaps help to decide the question (now in dispute among anatomists) whether the lemurs are really Primates, or form an altogether distinct and isolated order of mammalia.
Insectivora.—This order is represented by comparatively few forms in the tertiary beds, and these are all very different from existing types. In the Upper Miocene of Dakota are found remains indicating two extinct genera, Lepictis and Ictops. In the Miocene of Colorado, Professor Cope has recently discovered four new genera, Isacis; allied to the preceding, but as large as a Mephitis or skunk; Herpetotherium, near the moles; Embasis, more allied to the shrews; and Dommina, of uncertain affinities. Two others have been found in the Eocene of Wyoming; Amomys, having some resemblance to hedgehogs and to the Eastern Tupaia; and Washakius, of doubtful affinities.