Deer are represented by a single species of Cervus in the Pliocene, while two extinct genera, Leptomeryx and Merycodus, are found in the Miocene deposits, the latter indicating a transition between camels and deer. Two other genera, Hypisodus and Hypertragulus, of very small size, are said by Professor Cope to be allied to the Tragulidæ and to Leptomeryx.
The Bovidæ, or hollow-horned ruminants, are only represented in the Newer Pliocene by a single species of an extinct genus, Casoryx, said to be intermediate between antelopes and deer.
We now come to an exclusively American family, the Oreodontidæ, which consisted of small animals termed by Dr. Leidy, "ruminating hogs," and which had some general structural resemblances to deer and camels. They abounded in North America during the Pliocene, and especially during the Miocene epoch, no less than six genera and twenty species having been discovered. Merychus contains the Pliocene forms; while Oreodon, Eporeodon, Merychochœrus, Leptauchenia, and Agriochœrus are Miocene. The last genus extends back into the Eocene period, and shows affinity to the European Anoplotheridæ of the same epoch.
Proboscidea.—The Elephantidæ are only represented in America by one species of Mastodon and one of Elephas, in the Newer Pliocene deposits. In the Older Pliocene, Miocene, and Upper Eocene, no remains of this order have been found; and in 1869, Dr. Leidy remarked on the small average size of the extinct North American mammalia, which were almost all smaller than their living analogues. Since then, however, wonderful discoveries have been made in deposits of Middle Eocene age in Wyoming and Colorado, of a group of huge animals not only rivalling the elephants in size, but of so remarkable and peculiar a structure as to require the formation of a new order of mammals—Dinocerata—for their reception.
This order consists of animals with generalised Ungulate and Proboscidean affinities. The lower jaw resembles that of the hippopotamus; they had five toes on the anterior feet and four on the posterior; three pairs of horns, the first pair on the top of the head, large and perhaps palmated, the second pair above the eyes, while the third and smallest stood out sideways on the snout. They had enormous upper canines, of which the roots entered the middle horn cores, no upper incisors, and small molars. Professor Marsh believes that they had no trunk. The remains discovered indicate four genera, Dinoceras (3 sp.), Tinoceras (2 sp.), Uintatherium (1 sp.), and Eobasileus (2 sp.). Many other names have been given to fragments of these animals, and even those here given may not be all distinct.
Another new order, Tillodontia, recently established by Professor Marsh, is perhaps yet more remarkable in a zoological point of view, since it combines the characters of Carnivora, Ungulata, and Rodents. These animals have been formed into two families, Tillotheridæ and Stylinodontidæ; and three genera, Tillotherium, Anchippodus, and Stylinodontia. All are from the Eocene of Wyoming and New Jersey. Perhaps to these must be added Elotherium from the Miocene of Dakota, the other forms being all Eocene. They were mostly animals of small size, between that of the capybara and tapir. The skull resembled in form that of a bear; the molar teeth were of Ungulate type, and the incisors like those of a Rodent; but the skeleton was more that of the Ursidæ, the feet being plantigrade. Professor Cope has since described three new genera from the Eocene of New Mexico, Ectoganus, Calamodon, and Esthonyx, comprising seven species allied to Tillotherium and Anchippodus, and having also relations, as Professor Cope believes, with the South American Toxodontidæ.
Rodentia.—This order is represented in the Pliocene by a beaver, a porcupine, and an American mouse (Hesperomys), all extinct species of living genera, the Hystrix being an Old World type; and Professor Cope has recently described Panolax, a new genus of hares from the Pliocene of New Mexico. The Miocene deposits have furnished an extinct genus allied to the hares—Palæolagus; one of the squirrel family—Ischyromys; a small extinct form of beaver—Palæocastor; and an extinct mouse—Eumys. The Eocene strata of Wyoming have lately furnished two extinct forms of squirrel, Paramys and Sciuravus; and another of the Muridæ (or mouse family), Mysops.
Cetacea.—Numerous remains of dolphins and whales, belonging to no less than twelve genera, mostly extinct, have been found in the Miocene deposits of the Atlantic and Gulf States, from New Jersey to South Carolina and Louisiana; while seven genera of the extinct family, Zeuglodontidæ, have been found in Miocene and Eocene beds of the same districts. Some remains associated with these are doubtfully referred to the Seal family (Phocidæ) among the Carnivora.
Edentata.—Till quite recently no remains of this order have occurred in any North American deposits below the Post-Pliocene; but in 1874 Prof. Marsh described some remains allied to Megalonyx and Mylodon, from the Pliocene beds of California and Idaho, and forming a new genus, Morotherium. As these remains have only occurred to the west of the Rocky Mountains, and in Pliocene deposits whose exact age is not ascertained, they hardly affect the remarkable absence of this group from the whole of the exceedingly rich Tertiary deposits in all other parts of North America.
General Relations of the extinct Tertiary Mammalia of North America and Europe.—Having now given a sketch of the extinct Mammalia which inhabited Europe and North America during the Tertiary period, we are enabled by comparing them, to ascertain their relations to each other, and to see how far they elucidate the problem of the birth-place and subsequent migrations of the several families and genera. We have already pointed out the remarkable features of the Quaternary (or Post-Pliocene) fauna of North America, and now proceed to discuss that of the various Tertiary periods, which is closely connected with the extinct fauna of Europe.