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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.
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 PostPliocene 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.
Far back in the Triassic coal of North Carolina has been found the jaw of a small mammal (Dromotherium), the teeth of which somewhat resemble those of the Australian Myrmecobius, and may belong either to the Insectivora or Marsupials; if indeed, at that early period these orders were differentiated.
Carnivora.—The most ancient forms of this order are some remains found in the Middle Eocene of Wyoming, and others recently described by Professor Cope (1875) from the Eocene of New Mexico, of perhaps earlier date. The former consist of three genera, Patriofelis, Vintacyon, and Sinopa, animals of large size but which cannot be classed in any existing family; and two others, Mesonyx and Synoplotherium, believed by Mr. Cope to be allied to Hyonodon. The latter consist of four genera, — Oxyæna, consisting of several species, some as large as a jaguar, was allied to Hymnodon and Pterodon ; Pachycena, allied to the last; Prototomus, allied to Amphicyon and the Viverridæ; and Limnocyon, a civet-like carnivore with resemblances to the Canidæ.
In the Miocene formations we find the Feline type well developed. The wonderful Machairodus, which in Europe lived down to Post-Pliocene times, is found in the Upper Miocene of Dakota ; and perfect crania have been discovered, showing that the chin was lengthened downwards to receive and protect the enormous canines. Dinyctis was allied both to Machairodus and to the weasels. Three new genera have been lately described by Professor Cope from the Miocene of Colorado,-Bunælurus, with characters of both cats and weasels ; Daptophilus, allied to Dinyctis; and Hoplophoneus, more allieä to Machairodus. The Canidæ are represented by Amphicyon, which occurs in deposits of the same age in Europe ; and by Canis, four species of which genus are recorded by Professor Cope from the Miocene of Colorado, and it also occurs in the Pliocene. The Hycenodon is represented by three species in the Miocene of Dakota and Colorado. It occurs
also in the European Miocene and Upper Eocene formations, and constitutes a distinct family Hyænodontidæ, allied, according to Dr. Leidy, to wolves, cats, hyænas and weasels. The Ursidæ are represented by only one species of an extinct genus, Leptarchus, from the Pliocene of Nebraska. From the Pliocene of Colorado, Prof. Cope has recently described Tomarctos, as a "short-faced type of dog;" as well as species of Canis and Martes.
Ungulata.—The animals belonging to this order being usually of large size and accustomed to feed and travel in herds, are liable to wholesale destruction by floods, bogs, precipices, drought or hunger. It is for these reasons, probably, that their remains are almost always more numerous than those of other orders of mammalia. In America they are especially abundant; and the number of new and intermediate types about whose position there is much difference of opinion among Palæontologists, renders it very difficult to give a connected summary of them with any approach to systematic accuracy.
Beginning with the Perissodactyla, or odd-toed ungulates, we find the Equine animals remarkably numerous and interesting. The true horses of the genus Equus, so abundant in the PostPliocene formations, are represented in the Pliocene by several ancestral forms. The most nearly allied to Equus is Pliohippus, consisting of animals about the size of an ass, with the lateral toes not externally developed, but with some differences of dentition. Next come Protohippus and Hipparion, in which the lateral toes are developed but are small and functionless. Then we have the allied genera, Anchippus, Merychippus, and Hyohippus, related to the European Hippotherium, which were all still smaller animals, Protohippus being only 21 feet high. In the older deposits we come to a series of forms, still unmistakably equine, but with three or more toes used for locomotion and with numerous differentiations in form, proportions, and dentition. These constitute the family Anchitheridæ. In the Miocene we have the genera Anchitherium (found also in the European Miocene), Michippus and Mesohippus, all with three toes on each foot, and about the size of a sheep or large goat. In the Eocene of
Utah and Wyoming, we get a step further back, several species having been discovered about the size of a fox with four toes in front and three behind. These form the genus Orohippus, and are the oldest ancestral horse known. Prof. Marsh points out the remarkably perfect series of forms in America, which, beginning with this minute ancient type, is gradually modified by gaining increased size, increased speed by concentration of the limb-bones, elongation of the head and neck, the canine teeth decreased in size, the molars becoming longer and being coated with cementtill we at last come to animals hardly distinguishable, specifically, from the living horse.
Allied to these, are a series of forms showing a transition to the tapirs, and to the Palæotherium of the European Eocene. In the Pliocene we have Parahippus; in the Miocene Lophiodon, found in the same formation and in the Eocene of Europe, and allied to the tapir; and in the Eocene, Palæosyops, as large as a rhinoceros, which had large canines and was allied to the tapir and Palæotherium ; Limnohyus, forming the type of a family Limnohyidæ, which included the last genus and some others mentioned further on; and Hyrachyus, allied to Lophiodon, and to Hyracodon an extinct form of rhinoceros. Besides these we have Lophiotherium (also from the Eocene of Europe); Diplacodon allied to Limnohyus, but with affinities to modern Perissodactyla and nearly as large as a rhinoceros; and Colonoceras, also belonging to the Limnohyidæ, an animal which was the size of a sheep, and had divergent protuberances or horns on its nose. A remarkable genus, Bathmodon, lately described by Professor Cope, and of which five species have been found in the Eocene of New Mexico and Wyoming, is believed to form the type of a new family, having some affinity to Palæosyops and to the extinct Brontotheridæ. It had large canine tusks but no horns.
The Rhinocerotidæ are represented in America by the genus Rhinoceros in the Pliocene and Miocene, and by Aceratherium and Hyracodon in the Miocene. Both the latter were hornless, and Hyracodon was allied to the Eocene Hyrachyus, one of the Lophiodontidæ. In the Eocene and Miocene deposits of Utah, and Oregon, several remarkable extinct rhinoceroses have been