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 (Anserinoe) or Ducks (Anatidoe); 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.

[Illustration:  Fig. 227.—­Skull of Odontopteryx toliapicus restored.  (After Owen.)]

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