Fig. 209.—Teeth of Iguanodon Mantellii. Wealden, Britain. at from thirty to fifty feet, the thigh-bone in large examples measuring nearly five feet in length, with a circumference of twenty-two inches in its smallest part. With the strong and massive hind-limbs are associated comparatively weak and small fore-limbs; and there seems little reason to doubt that the Iguanodon must have walked temporarily or permanently upon its hind-limbs, after the manner of a Bird. This conjecture is further supported by the occurrence in the strata which contain the bones of the Iguanodon of gigantic three-toed foot-prints, disposed singly in a double track. These prints have undoubtedly been produced by some animal walking on two legs; and they can hardly, with any probability, be ascribed to any other than this enormous Reptile. Closely allied to the Iguanodon is the Hadrosaurus of the American Cretaceous, the length of which is estimated at twenty-eight feet. Iguanodon does not appear to have possessed any integumentary skeleton; but the great Hylœosaurus of the Wealden seems to have been furnished with a longitudinal crest of large spines running down the back, similar to that which is found in the comparatively small Iguanas of the present day. The Megalosaurus of the Oolites continued to exist in the Cretaceous period; and, as we have previously seen, it was carnivorous in its habits. The American Lœlaps was also carnivorous, and, like the Megalosaur, which it very closely resembles, appears to have walked upon its hind-legs, the fore-limbs being disproportionately small.
Another remarkable group of Reptiles, exclusively confined to the Cretaceous series, is that of the Mosasauroids, so called from the type-genus Mosasaurus. The first species of Mosasaurus known to science was the M. Camperi (fig. 210), the skull of which—six feet in length—was
Fig. 210.—Skull of Mosasaurus Camperi, greatly reduced. Maestricht Chalk. discovered in 1780 in the Maestricht Chalk at Maestricht. As this town stands on the river Meuse, the name of Mosasaurus ("Lizard of the Meuse") was applied to this immense Reptile. Of late years the remains of a large number of Reptiles more or less closely related to Mosasaurus, or absolutely belonging to it, have been discovered in the Cretaceous deposits of North America, and have been described by Professors Cope and Marsh. All the known forms of this group appear to have been of large size—one of them, Mosasaurus princeps, attaining the length of seventy-five or eighty feet, and thus rivalling the largest of existing Whales in its dimensions. The teeth in the "Mosasauroids" are long, pointed, and slightly curved; and instead of being sunk in distinct sockets, they are firmly amalgamated with the jaws, as in modern Lizards. The palate also carried teeth, and the lower jaw was so constructed as to allow of the mouth being opened to an immense width, somewhat as in the living Serpents. The body was long and snake-like, with a very long tail, which is laterally compressed, and must have served as a powerful swimming-apparatus. In addition to this, both pairs of limbs have the bones connecting them with the trunk greatly shortened; whilst the digits were enclosed in the integuments, and constituted paddles, closely resembling in structure the "flippers" of Whales and Dolphins. The neck is sometimes moderately long, but oftener very short, as the great size and weight of the head would have led one to anticipate. Bony plates seem in some species to have formed an at any rate partial covering to the skin; but it is not certain that these integumentary appendages were present in all. Upon the whole, there can be no doubt but that the Mosasauroid Reptiles—the true "Sea-serpents" of the Cretaceous period—were essentially aquatic in their habits, frequenting the sea, and only occasionally coming to the land.
The "Mosasauroids" have generally been regarded as a greatly modified group of the Lizards (Lacertilia). Whether this reference be correct or not—and recent investigations render it dubious—the Cretaceous rocks have yielded the remains of small Lizards not widely removed from existing forms. The recent order of the Chelonians is also represented in the
Fig. 211.—Carapace of Chelone Benstedi. Lower Chalk. (After Owen.) Cretaceous rocks, by forms closely resembling living types. Thus the fresh-water deposits of the Wealden have yielded examples of the "Terrapins" or "Mud-Turtles" (Emys); and the marine Cretaceous strata have been found to contain the remains of various species of Turtles, one of which is here figured (fig. 211). No true Serpents (Ophidia) have as yet been detected in the Cretaceous rocks; and this order does not appear to have come into existence till the Tertiary period. Lastly, true Crocodiles are known to have existed in considerable numbers in the Cretaceous period. The oldest of these occur in the fresh-water deposit of the Wealden; and they differ from the existing forms of the group in the fact that the bodies of the vertebræ, like those of the Jurassic Crocodiles, are bi-concave, or hollowed out at both ends. In the Greensand of North America, however, occur the remains of Crocodiles which agree with all the living species in having the bodies of the vertebræ in the region of the back hollowed out in front and convex behind.
Birds have not hitherto been shown, with certainty, to have existed in Europe during the Cretaceous period, except in a few instances in which fragmentary remains belonging to this class have been discovered. The Cretaceous deposits of North America have, however, been shown by Professor Marsh to contain a considerable number of the remains of Birds, often in a state of excellent preservation. Some of these belong to Swimming or Wading Birds, differing in no point of special interest from modern birds of similar habits. Others, however, exhibit such extraordinary peculiarities that they merit more than a passing notice. One of the forms in question constitutes the genus Ichthyornis of Marsh, the type-species of which (I. Dispar) was about as large as a Pigeon. In two remarkable respects, this singular Bird differs from all known living members of the class. One of these respects concerns the jaws, both of which exhibit the Reptilian character of being armed with numerous small pointed teeth (fig. 212, a), sunk in distinct sockets. No existing bird possesses teeth; and this character forcibly recalls the Bird-like Pterosaurs, with their toothed jaws. Ichthyornis, however, possessed fore-limbs constructed strictly on the type of the "wing" of the living Birds; and it cannot, therefore, be separated from this class. Another extraordinary peculiarity of Ichthyornis is, that the bodies of the vertebrie (fig. 212, c) were bi-concave, as is the case with many extinct Reptiles and almost all Fishes, but as does not occur in any living Bird. There can be little doubt that Ichthyornis was aquatic in its habits, and that it lived principally upon fishes; but its powerful wings at the same time indicate that it was capable of prolonged flight. The tail of Ichthyornis has, unfortunately, not been discovered; and it is at present impossible to say whether this resembled the tail of existing Birds, or whether it was elongated and composed of separate vertebræ, as in the Jurassic Archœopteryx.
Still more wonderful than Ichthyornis is the marvellous bird described by Marsh under the name of Hesperornis regalis. This presents us with a gigantic diving bird, somewhat resembling the existing "Loons" (Colymbus), but agreeing with Ichthyornis in having the jaws furnished with conical, recurved, pointed teeth (fig. 212, b). Hence these forms are grouped together in a new sub-class, under the name of Odontornithes or "Toothed Birds." The teeth of Hesperornis (fig. 212, d) resemble those of Ichthyornis in their general form; but instead of being