Fig. 227.—Skull of Odontopteryx toliapicus restored. (After Owen.) the jaw itself, with which they are continuous, and which were probably encased by extensions of the horny sheath of the bill. These tooth-like processes are of two sizes, the larger ones being comparable to canines; and they are all directed forwards, and have a triangular or compressed conical form. From a careful consideration of all the discovered remains of this bird, Professor Owen concludes that "Odontopteryx was a warm-blooded feathered biped, with wings; and further, that it was web-footed and a fish-eater, and that in the catching of its slippery prey it was assisted by this Pterosauroid armature of its jaws." Upon the whole, Odontopteryx would appear to be most nearly related to the family of the Geese (Anserinœ) or Ducks (Anatidœ); but the extension of the bony substance of the jaws into tooth-like processes is an entirely unique character, in which it stands quite alone.
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
Fig. 228.—Zeuglodon cetoides. A, Molar tooth of the natural size; B, Vertebra, reduced in size. From the Middle Eocene of the United States. (After Lyell.) 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 cetoides of Owen, which attained a length of seventy feet. Remains of these gigantic Whales are very common in the "Jackson Beds" of the Southern United States. So common are they that, according to Dana, "the large vertebræ, some of them a foot and a half long and a foot in diameter, were formerly so abundant over the country, in Alabama, that they were used for making walls, or were burned to rid the fields of them."
The great and important order of the Hoofed Quadrupeds (Ungulata) is represented in the Eocene by examples of both of its two principal sections—namely, those with an uneven number of toes (one or three) on the foot (Perissodactyle Ungulates), and those with an even number of toes (two or four) to each foot (Artiodactyle Ungulates). Amongst the Odd-toed Ungulates, the living family of the Tapirs (Tapirdœ) is represented by the genus Coryphodon of Owen. Nearly related to the preceding are the species of Palœotherium, which have a historical interest as being amongst the first of the Tertiary Mammals investigated by the illustrious Cuvier. Several species of Palœothere are known, varying greatly in size, the smallest being little bigger than a hare, whilst the largest must have equalled a good-sized horse in its dimensions. The species of Palœotherium appear to have agreed with the existing Tapirs in possessing a lengthened and flexible nose, which formed a short proboscis or trunk (fig. 229), suitable as an instrument for stripping off the foliage of trees—the characters of the molar teeth showing them to have been strictly herbivorous in their habits. They differ, however, from the Tapirs, amongst other characters, in the fact that both the fore and the hind feet possessed three toes each; whereas in the latter there are four toes on each fore-foot, and the hind-feet alone are three-toed. The remains of Palœotheria have been found in such abundance in certain localities as to show that these animals roamed in great herds over the fertile plains of France and the south of England during the later portion of the Eocene period. The accompanying illustration (fig. 229) represents the notion which the great Cuvier was induced by his researches to form as to the outward appearance of Palœotherium magnum. Recent discoveries,
Fig. 229.—Outline of Palœotherium magnum, restored. Upper Eocene, Europe. (After Cuvier.) however, have rendered it probable that this restoration is in some important respects inaccurate. Instead of being bulky, massive, and more or less resembling the living Tapirs in form, it would rather appear that Palœotherium magnum was in reality a slender, graceful, and long-necked animal, more closely resembling in general figure a Llama, or certain of the Antelopes.
The singular genus Anchitherium forms a kind of transition between the Palœotheria and the true Horses (Equidœ). The Horse (fig. 230, D) possesses but one fully-developed toe to each foot, this being terminated by a single broad hoof, and representing the middle toe—the third of the typical five-fingered or five-toed limb of Quadrupeds in general. In addition, however, to this fully-developed toe, each foot in the horse carries two rudimentary toes which are concealed beneath the skin, and are known as the "splint-bones." These are respectively the second and fourth toes, in an aborted condition; and the first and fifth toes are wholly wanting. In Hipparion (fig. 230, C), the foot is essentially like that of the modern Horses, except that the second and fourth toes no longer are mere "splint-bones," hidden beneath the skin; but have now little hoofs, and hang freely, but uselessly, by the side of the great middle toe, not being sufficiently developed to reach the ground. In Anchitherium, again (fig. 230, B), the foot is three-toed, like that of Hipparion; but the two lateral toes (the second and fourth) are so far developed that they now reach the ground. The first digit (thumb or great toe) is still wanting; as also is the fifth digit (little finger or little toe).