smooth, and the fang entirely absorbed in consequence of the pressure of a successional tooth.

In other specimens in the Museum, the crown is reduced to a mere plate of coarse dentine, flat above, and slightly concave beneath; a proof that the teeth remained attached to the alveolar process, till the fang was entirely absorbed, and

the new crown ready to pierce the gum, and take part in the process of mastication. The removal of the fang by absorption is manifest in almost every specimen, as I pointed out in my earliest memoir on the subject; and the effect of this process may be traced through every gradation, in like manner as the destruction of the crown of which I have already spoken. Now it is the power of perfect mastication possessed by the colossal reptile to which these teeth belonged, and of which we have such unquestionable proofs in the specimens before us, that is so marvellous a fact to the zoologist, who reflects that the essenFLAT, AND THE FANG ABSORBED. tial conformation of the jaws and teeth in the class Reptilia, as exemplified in existing forms, forbids the supposition that such a structure as is implied by a power of mastication equal to that enjoyed by the herbivorous mammalia, was associated with reptilian organization. As we have already remarked, no living reptiles can masticate their food; the insects or vegetables on which they feed are seized by the tongue or teeth, and swallowed whole; so that a moveable covering to the jaws, similar to the lips and cheeks of the mammalia, is not necessary, either for confining substances subjected to the action of the teeth as organs of mastication, or for the purpose of seizing or retaining their prey. The herbivorous reptiles gnaw off the vegetables on which they subsist, but do not chew them; consequently their teeth, when worn by use,

LIGN. 52.

(nat. size.)

1 "Philos. Trans." 1825.

present a broken or chipped appearance, but not a smooth, flat-worn surface as in the Iguanodon.

INTIMATE STRUCTURE OF THE TEETH OF THE IGUANODON.Mr. Tomes, F.R.S., whose original and profound microscopic investigations have shed important light on the intimate structure of osseous and dental tissues, has favoured me with the following notes on the organization of the teeth of the Iguanodon.

"The teeth of the Iguanodon present structural peculiarities which, with our present experience in dental tissues, can be confounded with those of no other animal. The enamel is reptilian in character,—that is, it exists as a thin layer, not exceeding the 200th of an inch at the thickest part, and in many places is even thinner; and then it has the usual structureless appearance, with faint wavy markings, in contour lines with the surface of the dentine. Here and there, however, faint lines may be seen proceeding from the surface of the dentine to that of the enamel, which, together with the disposition to break in the direction of the lines, indicates pretty surely the existence of fibres.

"The dentine of the tooth of the Iguanodon is very remarkable when considered in connexion with the position of the animal in the scale of vertebrata. The dentinal tubes are well marked. They make a bold double curve in their passage from the pulp-cavity to the surface, in addition to the minute undulations which characterize them in every part of their course, and in no part are they free from short, minute, ragged, hair-like branches, which in a thick section give a confused appearance to the tissue. In a longitudinal section of a tooth, the tubes have a diameter varying from the 10,000th to the 15,000th of an inch, which is preserved to near their termination at the surface of the enamel, into which tissue a few are continued a perceptible distance. The pulpcavity is marked by a series of indentations at tolerably regular intervals. From the recesses of these, vascular canals proceed into the substance of the dentine, and follow the course of the dentinal tubes, till near the periphery of the tooth, when they terminate in dilated extremities, or turn and follow a parallel course till they regain the pulp cavity. As the tooth becomes worn, these canals are filled by a transparent, almost structureless tissue, in the manner described in my paper on the teeth of rodents.1 It should also be remarked that the dentinal tubes are connected with them through their branches only, and by these but sparingly. Professor Owen, in his account of the tooth of the Iguanodon, (Odontography,' p. 251,) compares the vascular or medullary canals to those which occur in the inner dentine of the tooth of the Megatherium. In the latter, however, the canals are far more numerous than in the Iguanodon, and, moreover, the dentine in which they occur is dissimilar. In it the dentinal tubes are so much interfered with by the very numerous vascular canals that they become irregular, and indeed can scarcely be called tubes; they are irregular cells, minute, and

1 "Philos. Trans." 1850.

without definite arrangement. In the Iguanodon, on the contrary, the dentinal tubes and their branches are well marked, and definitely arranged.

"On careful comparison it will be found that the dentine of the teeth of this great vegetable-feeding reptile pretty closely resembles that of many of the larger herbivorous mammalia, and more especially the ruminants. In these we find the dentine traversed, though less abundantly, by similar vascular canals. In the American Tapir, and also in the Solipedes, a similar condition is observable. In mammalian dentine the parietes of the dentinal tubes are well marked, especially in the ruminants, where they are extremely thick; this cannot be said to be the case in the Iguanodon, for although the parietes may be seen in a favourable section, yet they are by no means so distinct or so thick; neither, indeed, are the dentinal tubes themselves so large as in the great mammalian herbivora. So far as my own experience goes, the presence of vascular canals in the substance of the dentine as a constant character, is confined to the teeth of the vegetable feeders.

"Professor Owen has described, (Odontography,' p. 252,) a third substance in the tooth of the Iguanodon. He says, The remains of the pulp in the contracted cavity of the completely formed tooth, are converted into a dense but true osseous substance, characterized by minute elliptical radiated cells, whose long axis is parallel with the plane of the concentric lamella which surround the few and contracted medullary canals in this substance.' I have seen the concentric lamellæ in the situation described by Professor Owen, and these have been perforated by straggling, irregular, dentinal tubes, but I have failed to observe in this or in any other part of the tooth elliptical radiate cells,—in other words, bone or cement lacunæ.

"In the central part of these teeth, a dark brown matter is commonly seen. This is for the most part composed of small, oval, ferruginouslooking bodies about the size of bone lacunæ, which are surrounded by imperfectly formed crystalline matter. This brown substance occupies the pulp-cavity, and often extends a short distance into the vascular canals. In a thick section it might on a casual inspection be taken for cementum, but a more careful observation would at once show it to be a mere product of fossilization.

"If I had to describe the tooth of the Iguanodon from its tissues in a few words, I should say it was a tooth having herbivorous (mammalian?) dentine, with reptilian enamel."

LOWER JAW OF THE IGUANODON.-Ligns. 54 and 55.—The importance of discovering the peculiar construction of the maxillary organs which had impressed such anomalous characters on the teeth of a reptile, as to impart to those instruments so striking a resemblance to the incisors of herbivorous mammalia as to mislead the most eminent anatomist of modern times, could not be estimated too highly; and for many years, my curiosity and interest were painfully excited. by the desire of solving the mystery in which the subject

was involved; but nearly a quarter of a century passed by ere that privilege was attained.

Although the specimen I am about to describe does not belong to the national collection, yet it throws so much light on the subject under review, and imparts such additional interest to the objects before us, that I feel assured the intelligent reader will not consider the following somewhat minute description of the first discovered portion of the lower jaw of the Iguanodon, as irrelevant to the immediate purpose of this volume.

LOWER JAW OF THE IGUANODON.'-Lign. 53 and 54.—In the deltas and estuaries of rivers which flow through countries of varied geological structure, we naturally expect to find the remains of terrestrial vertebrated animals that have been transported by the currents from far distant lands, in a more or less mutilated state; the skeletons broken up-the bones dissevered, fractured, and waterworn-the teeth detached from the jaws and dispersed-and all these separated parts promiscuously imbedded in the mud, silt, and sand of the delta, and intermingled with the debris of the flora of the country, and the remains of fishes, mollusks, and crustaceans, that inhabited the fresh water, or were denizens of the adjacent sea. Such, as we bave already pointed out, is the condition in which the bones and teeth of oviparous quadrupeds are found in the Wealden formation, and hence the difficulty of obtaining satisfactory evidence of the form and structure of the extinct reptiles whose relics are so abundant in some of the deposits.

To this cause may be ascribed the remarkable fact, that although many hundred teeth, belonging to several genera of saurians, have been collected from these fluviatile strata, scarcely a portion of the cranium, and but a few fragments of the jaws, have been discovered. Every relic of this kind is consequently in the highest degree interesting, and it was,

1 The following account of the maxillary organ of the Iguanodon is an abstract of my Memoir, "On the Structure of the Jaws and Teeth of the Iguanodon," communicated to the Royal Society in May, 1848, and published in the " Philosophical Transactions" of the same year. The Royal Medal of the Society was awarded to the author for that communication.

R

therefore, most gratifying to me to learn that at length a considerable portion of the lower jaw, with teeth, of an Iguanodon, had been obtained from the quarry near Cuckfield, in Sussex, in which the teeth of this colossal herbivorous lizard were first discovered.

In a communication addressed to the Royal Society in 1841,' I figured and described a fragment of the lower jaw of a small reptile as, probably, that of a young Iguanodon, and the anatomical considerations which seemed to support that intérpretation were fully detailed. But although, from the form and the mode of implantation of the fangs, which are the only vestiges of the teeth in the specimen, and the position of the germs of the successional ones, this inference appeared to me highly probable; yet, as the crowns of the teeth were wanting, the presumed generic identity could not be established, since it was possible the fossil might belong to the Hylæosaurus, or to some unknown genus of reptiles whose bones occur in the Wealden deposits, as afterwards proved to be the case.

But the specimen to which I now solicit attention is the right side of the lower jaw of an adult animal, with two successional teeth in place, and the germ of a third, and the alveoli or sockets of seventeen or eighteen mature molars, and is the first indisputable portion of the jaw of the Iguanodon which has hitherto been brought to light; and although, from the absence of mature teeth, and of the articular portion of the jaw, this specimen does not afford a complete solution of the problem discussed in the preceding pages, it possesses characters sufficiently definite and intelligible to throw important light on the structure and functions of the dental organs of the Iguanodon; and it has also enabled me to determine the nature of a portion of the left upper maxillary bone, collected many years since, and now in the British Museum, but which I was previously unable to interpret.

Before entering upon the description of this unique and most interesting fossil, I must express my warmest acknowledgments to Captain Lambart Brickenden, of Warminglid, Sussex (now of Elgin, Scotland), by whom it was discovered, and skilfully extricated from the sandstone in which it was

1" Philos. Trans." 1841, p. 131.