in the same quarries, that we are nearly as well acquainted with the form and dimensions of its limbs, as if they had been found together in a single block of stone. From the size and proportions of these bones, as compared with existing Lizards, Cuvier concludes the Megalosaurus to have been an enormous reptile, measuring from forty to fifty feet in length, and partaking of the structure of the Crocodile and Monitor. As the femur and tibia measure nearly three feet each, the entire hind-leg must have attained a length of nearly two yards: a metatarsal bone, thirteen inches long, indicates a corresponding length in the foot.* The bones of the thigh and leg are not solid at the centre, as in Crocodiles, and other aquatic quadrupeds, but have large medullary cavities, like the bones of terrestrial animals. We learn from this circumstance, added to the character of the foot, that the Megalosaurus lived chiefly upon the land. In the internal condition of these fossil bones, we see the same adaptation of the skeleton to its proper element, which now distinguishes the bones of terrestrial, from those of aquatic Saurians. In the Ichthyosauri and Plesiosauri, whose paddles were calculated exclusively to move in water, even the largest bones of the arms and legs were solid throughout. Their weight would in no way have embarrassed their action in the fluid medium they inhabited; but in the huge Megalosaurus, and still more gigantic Iguanodon, which are shown by the character of their feet to have been fitted to move on land, the larger bones of the legs were diminished in weight, by being internally hollow, and having their cavities filled with the light material of 1826, saw fragments of a jaw, containing teeth, and of some other bones of Megalosaurus, in the museum at Besançon, from the oolite of that neighbourhood. * See Geol. Trans. 2d series, Vol. 3, p. 427, Pl. 41. + I learn from Mr. Owen that the long bones of land Tortoises have a close cancellous internal structure, but not a medullary cavity. VOL. I.-16 marrow, while their cylindrical form tended also to combine this lightness with strength.* The form of the teeth shows the Megalosaurus to have been in a high degree carnivorous: it probably fed on smaller reptiles, such as Crocodiles and Tortoises, whose remains abound in the same strata with its bones. It may also have taken to the water in pursuit of Plesiosauri and fishes.t The most important part of the Megalosaurus yet found, consists of a fragment of the lower jaw, containing many * The medullary cavities in the fossil bones of the Megalosaurus, from Stonesfield, are usually filled with calcareous spar. In the Oxford Museum there is a specimen from the Wealden fresh-water formation at Langton, near Tunbridge Wells, which is perhaps unique amongst organic remains: it presents the curious fact of a perfect cast of the interior of a large bone, apparently the femur of a Megalosaurus, exhibiting the exact form and ramifications of the marrow, whilst the bone itself has entirely perished. The substance of this cast is fine sand, cemented by oxide of iron, and its form distinctly represents all the minute reticulations, with which the marrow filled the intercoluminations of the cancelli, near the extremity of the bone. It exhibits also casts of the perforations along the internal parietes, whereby the vessels entered obliquely from the exterior of the bone, to communicate with the marrow. A mould of the exterior of the same bone has been also formed by the sandstone in which it was imbedded; hence although the bone itself has perished, we have precise representations both of its external form and internal cavities, and a model of the marrow that filled this femur, nearly as perfect as could be made by pouring wax into an empty marrow bone, and corroding away the bone with acid. The sand which formed this cast must have entered the medullary cavity by a fracture across the other extremity of the bone, which was wanting in the specimen. From this natural preparation of ancient anatomy we learn that the disposition of marrow, and its connexion with the reticulated extremities of the interior of the femur, were the same in these gigantic Lizards of a former world, as in medullary cavities of existing species. + Mr. Broderip informs me that a living Iguana (I. Tuberculata,) in the gardens of the Zoological Society of London, in the summer of 1834, was observed frequently to enter the water, and swim across a small pond, using its long tail as the instrument of progression, and keeping its fore-feet motionless. teeth, (Pl. 23, Figs. 1'-2'.) The form of this jaw shows that the head was terminated by a straight and narrow snout, compressed laterally like that of the Delphinus Gangeticus. saurus. As in all animals, the jaws and teeth form the most characteristic parts, I shall limit my present observations to a few striking circumstances in the dentition of the MegaloFrom these we learn that the animal was a reptile, closely allied to some of our modern Lizards; and viewing the teeth as instruments for providing food to a carnivorous creature of enormous magnitude, they appear to have been admirably adapted to the destructive office for which they were designed. Their form and mechanism will best be explained by reference to the figures in Pl. 23.* In the structure of these teeth, (Pl. 23, Figs. 1, 2, 3,) we find a combination of mechanical contrivances analogous to those which are adopted in the construction of the knife, the sabre, and the saw. When first protruded above the gum, (Pl. 23, Figs. 1'. 2′.) the apex of each tooth presented a double cutting edge of serrated enamel. In this stage, its position and line of action were nearly vertical, and its *The outer margin of the jaw (Pl. 23, Fig. 17. 2′.) rises nearly an inch above its inner margin, forming a continuous lateral parapet to support the teeth on the exterior side, where the greatest support was necessary; whilst the inner margin (Pl. 23, Fig. 1') throws up a series of triangular plates of bone, forming a zig-zag buttress along the interior of the alveoli. From the centre of each triangular plate, a bony partition crosses to the outer parapet, thus completing the successive alveoli. The new teeth are seen in the angle between each triangular plate, rising in reserve to supply the loss of the older teeth, as often as progressive growth, or accidental fracture, may render such renewal necessary; and thus affording an exuberant provision for a rapid succession and restoration of these most essential implements. They were formed in distinct cavities, by the side of the old, teeth, towards the interior surface of the jaw, and probably expelled them by the usual process of pressure and absorption; insinuating themselves into the cavities thus left vacant. This contrivance for the renewal of teeth is strictly analogous to that which takes place in the dentition of many species of existing Lizards. form like that of the two-edged point of a sabre, cutting equally on each side. As the tooth advanced in growth, it became curved backwards, in the form of a pruning knife, (Pl. 23, Figs. 1. 2. 3.) and the edge of serrated enamel was continued downwards to the base of the inner and cutting side of the tooth, (Fig. 1, B. D.) whilst, on the outer side, a similar edge descended, but to a short distance from the point (Fig. 1, B. to C.) and the convex portion of the tooth (A.) became blunt and thick, as the back of a knife is made thick, for the purpose of producing strength. The strength of the tooth was farther increased by the expansion of its sides, (as represented in the transverse section, Fig. 4, A. D.) Had the serrature continued along the whole of the blunt and convex portion of the tooth, it would, in this position, have possessed no useful cutting power; it ceased precisely at the point, (C.) beyond which it could no longer be effective In a tooth thus formed for cutting along its concave edge, each movement of the jaw combined the power of the knife and saw; whilst the apex, in making the first incision, acted like the two-edged point of a sabre. The backward curvature of the full-grown teeth, enabled them to retain, like barbs, the prey which they had penetrated. In these adaptations, we see contrivances, which human ingenuity has also adopted, in the preparation of various instruments of art. In a former chapter (Ch. XIII.) I endeavoured to show that the establishment of carnivorous races throughout the animal kingdom tends materially to diminish the aggregate amount of animal suffering. The provision of teeth and jaws, adapted to effect the work of death most speedily, is highly subsidiary to the accomplishment of this desirable end. We act ourselves on this conviction, under the impulse of pure humanity, when we provide the most efficient instruments to produce the instantaneous, and most easy death, of the innumerable animals that are daily slaughtered for the supply of human food. SECTION X. IGUANO DON.* This As the reptiles hitherto considered appear from their teeth to have been carnivorous, so we find extinct species of the same great family, that assume the character and office of herbivora. For our knowledge of this genus, we are indebted to the scientific researches of Mr. Mantell. indefatigable historian of the Wealden fresh-water formation, has not only found the remains of the Plesiosaurus, Megalosaurus, Hylæosaurus,† and several species of Crocodiles and Tortoises in these deposites, of a period intermediate between the oolitic and cretaceous series, but has also discovered in Tilgate Forest the remains of the Iguanodon, a reptile much more gigantic than the Megalosaurus, and which, from the character of its teeth, appears to have been herbivorous. The teeth of the Iguanodon are so pre * Sce Pl. 1, Fig. 45, and Pl. 24; and Mantell's Geology of Sussex, and of the south-east of England. The Hylæosaurus, or Lizard of the Weald, was discovered in Tilgate Forest, in Sussex, in 1832. This extraordinary Lizard was probably about twenty five feet long. Its most peculiar character consists in the remains of a series of long, flat, and pointed bones, which seems to have formed an cnormous dermal fringe, like the horny spines on the back of the modern Iguana. These bones vary in length from five to seventeen inches, and in width from three to seven inches and a half at the base. Together with them were found the remains of large dermal bones, or thick scales which were probably lodged in the skin. The Iguanodon has hitherto been found only, with one exception, in the Wealden fresh-water formation of the south of England, (Pl. 1, section 22.) intermediate between the marine oolitic deposites of the Portland stone and those of the green-sand formation in the cetaceous series. The discovery, in 1834, (Phil. Mag. July, 1834, p. 77.) of a large proportion of the skeleton of one of these animals, in strata of the latter formation, in the quarries of Kentish Rag, near Maidstone, shows that the duration of this animal |