The known _Mammals_ of the Mesozoic period, as we have seen, are all of small size; and with one not unequivocal exception, they appear to be referable to the order of the Pouched Quadrupeds (_Marsupials_), almost the lowest group of the whole class of the Mammalia. In the Eocene rocks, on the other hand, numerous remains of Quadrupeds have been brought to light, representing most of the great Mammalian orders now in existence upon the earth, and in many cases indicating animals of very considerable dimensions. We are, in fact, in a position to assert that the majority of the great groups of Quadrupeds with which we are familiar at the present day were already in existence in the Eocene period, and that their ancient root-stocks were even in this early time separated by most of the fundamental differences of structure which distinguish their living representatives. At the same time, there are some amongst the Eocene quadrupeds which have a "generalised" character, and which may be regarded as structural types standing midway between groups now sharply separated from one another. The order of the _Marsupials_--including the existing Kangaroos, Wombats, Opossums, Phalangers, &c.--is poorly represented in deposits of Eocene age. The most celebrated example of this group is the _Didelphys gypsorum_ of the Gypseous beds of Montmartre, near Paris, an Opossum very nearly allied to the living Opossums of North and South America. No member of the _Edenates_ (Sloths, Ant-eaters, and Armadillos) has hitherto been detected in any Eocene deposit. The aquatic order of the _Sirenians_ (Dugongs and Manatees), with their fish-like bodies and tails, paddle-shaped forelimbs, and wholly deficient hind-limbs, are represented in strata of this age by remains of the ancient "Sea-Cows," to which the name of _Halitherium_ has been applied. Nearly allied to the preceding is the likewise aquatic order of the Whales and Dolphins (_Cetaceans_), in which the body is also fish-like, the hind-limbs are wanting, the fore-limbs are converted into powerful "flippers" or swimming-paddles, and the terminal extremity of the body is furnished with a horizontal, tail-fin. Many existing Cetaceans (such as the Whalebone Whales) have no true teeth; but others (Dolphins, Porpoises, Sperm Whales) possess simple conical teeth. In strata of Eocene age, however, we find a singular group of Whales, constituting the genus _Zeuglodon (fig. 228), in which the teeth differed from those of all existing forms in being of two kinds,--the front ones being conical incisors, whilst the back teeth or molars have serrated triangular crowns, and are inserted in the jaw by two roots. Each molar (fig. 228, A) looks as if it were composed of two separate teeth united on one side by their crowns; and it is this peculiarity which is expressed by the generic name (Gr. _zeugle_, a yoke; _odous_, tooth). The best-known species of the genus is the _Zeuglodon