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 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

The outer margin of the jaw (Pl. 23, Fig. 1'. 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 inuer 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.

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 further 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 twoedged 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 endea

voured 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.

IGUANODON.

As the reptiles hitherto considered appear from their teeth to have been carnivorous, so we find extinct species of the same great family, that assumed the character and office of herbivora. For our knowledge of this genus, we are indebted to the scientific researches of Mr. Mantell. This indefatigable historian of the Wealden fresh-water formation, has not only found

See Pl. 1, Fig. 45, and Pl. 24; and Mantell's Geology of Sussex, and of the South-east of England.

the remains of the Plesiosaurus, Megalosaurus, Hylæosaurus,* and several species of Crocodiles and Tortoises in these deposits, 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. teeth of the Iguanodon are so precisely similar, in the principles of their construction, to the teeth of the modern Iguana, as to leave no

The

*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 enormous 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, sec. 22.), intermediate between the marine oolitic deposits of the Portland stone and those of the greensand formation in the cretaceous 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, shews that the duration of this animal did not cease with the completion of the Wealden series. The individual from which this skeleton was derived had probably been drifted to sea, as those which afforded the bones found in the fresh-water depo

doubt of the near connection of this most gigantic extinct reptile with the Iguanas of our own time. When we consider that the largest living Iguana rarely exceeds five feet in length, whilst the congenerous fossil animal must have been nearly twelve times as long, we cannot but be impressed by the discovery of a resemblance, amounting almost to identity, between such characteristic organs as the teeth, in one of the most enormous among the extinct reptiles of the fossil world, and those of a genus whose largest species is comparatively so diminutive. According to Cuvier, the common Iguana inhabits all the warm regions of America: it lives chiefly upon trees, eating fruits, and seeds, and leaves. The female occasionally visits the water, for the purpose of laying in the sand its eggs, which are about the size of those of a pigeon.*

sits subjacent to this marine formation, had been drifted into an estuary. This unique skeleton is now in the museum of Mr. Mantell, and confirms nearly all his conjectures respecting the many insulated bones which he had referred to the Iguanodon.

* In the Appendix to a paper in the Geol. Trans. Lond. (N. S. Vol. III. Pt. 3) on the fossil bones of the Iguanodon, found in the Isle of Wight and Isle of Purbeck, I have mentioned the following facts, illustrative of the herbivorous habits of the living Iguana.

In the spring of 1829, "Mr. W. J. Broderip saw a living Iguana, about two feet long, in a hothouse at Mr. Miller's nursery gardens, near Bristol. It had refused to eat insects, and