some physiological deductions from their peculiar osteological. characters, we are at once struck with their remarkable deviation from all known types in the class Reptilia. In the Amblyrhynchi, the most exclusively vegetable feeders of the saurian order, the alveolar process, beset with teeth, is continued round the front of the mouth; the junction of the two rami of the lower jaw at the symphysis presenting no edentulous interval whatever, and the lips not being more produced than in other reptiles; for these creatures chip off and bruise their food, and cannot grind or masticate it: in the Iguanas, as previously shown, the same character exists. In the carnivorous saurians the teeth are also continued to the symphysial suture on each side. The extinct colossal lizards offer no exception to this rule; in the acrodont Mosasaurus of the Chalk, and in the thecodont Megalosaurus of the Oolite and Wealden, the jaws are armed with teeth round the anterior extremity. In short, the edentulous, expanded, scoop-shaped, procumbent, symphysis of the lower jaw of the Iguanodon, has no parallel among either existing or fossil reptiles, and we seek in vain for maxillary organs at all analogous, except among the herbivorous mammalia. The nearest approach is to be found in certain Edentata,—as for example in the Cholapus didactylus, or Two-toed Sloth,-in which the anterior part of the lower jaw is edentulous and much prolonged. The correspondence is still closer in the gigantic extinct Mylodon, in which the symphysis resembles the blade of a spade used by turf-diggers, and has no traces of incisive sockets; and were not this part of the jaw elevated vertically in front, and the two rami confluent, it would present the very counterpart of that of the Iguanodon.'

The great size and number of the vascular foramina distributed along the outer side of the dentary bone, and beneath the border of the symphysis, in the Iguanodon, and the magnitude of the anterior outlets which gave exit to the vessels

1 In the Mylodon Darwinii the rami of the lower jaw anterior to the teeth are contracted vertically, and converge to a longer and narrower symphysis, which is inclined forwards at a more open angle with the horizontal ramus, than in the Mylodon robustus, and therefore still more nearly approaches that of the Iguanodon. See Professor Owen on the Mylodon.

and nerves that supplied the front of the mouth, indicate the great development of the integuments and soft parts, with which the lower jaw was invested.

The sharp ridge bordering the deep groove of the symphysis, in which there are also several foramina, evidently gave attachment to the muscles and integuments of the under lip; while two deep pits for the insertion of the protractor muscles of the tongue, manifest the mobility and power of that organ. There are therefore strong reasons for supposing that the lip was flexible, and, in conjunction with the long fleshy prehensile tongue, constituted the instruments for seizing and cropping the leaves and branches, which, from the construction of the molars, we may infer constituted the chief food of the Iguanodon. The mechanism of the maxillary organs, as elucidated by recent discoveries, is thus in perfect harmony with the remarkable characters which rendered the first known teeth so enigmatical; and in the Wealden herbivorous reptile we have a solution of the problem, how the integrity of the type of organization peculiar to the class of cold-blooded vertebrata was maintained, and yet adapted, by simple modifications, to fulfil the conditions required by the economy of a gigantic terrestrial reptile, destined to obtain support exclusively from vegetable substances; in like manner as the extinct colossal herbivorous Edentata which flourished in South America, ages after the Country of the Iguanodon and its inhabitants had been swept from the face of the earth.

Thus in the unlimited production of successional teeth at every period of the animal's existence, in the mode of implantation of the teeth, and in the composite structure of the lower jaw, each ramus consisting of six distinct elements,the saurian type of organization is unequivocally manifest; while the intimate structure of the dental organs approaches that of the Sloths, and the subalternate arrangement and reversed position of the upper and lower series of teeth corresponds with that of the Ruminants. And again, the edentulous and prolonged symphysis, and the great development of the lower lip and the integuments of the jaws, as indicated by the size and number of the vascular foramina, present a striking analogy to the Edentata. They who doubt the correctness of this interpretation, should remember that

it is in this order of Mammalia that we find the nearest approach to the Reptilia: in the scaly covering of the skin, the imperfections of the dental system, the smallness of the brain, and the long continuance of the irritability of the muscular fibre after death; which are so many decadencies of organization, so to speak, that indicate a step towards that class, of which the Iguanodon appears to have been the highest type.

If the opinion previously advanced (ante, p. 260) be correct, that the anterior part of the spinal column consisted of convexo-concave, and the dorsal of plano-concave vertebræ, the adult Iguanodon must have approached in this part of its skeleton, as well as in its sacrum,-in its massive femora, with their large medullary cavities, trochanters, and condyles, and in its short and strong metatarsals and phalangeals, to that of the large herbivorous mammalia.

The position of the hinder limbs (the thighs and legs) in relation to the pelvis, cannot be accurately determined from the data at present obtained; but the form of the head and shaft of the femur, and the character of its articulations and processes, so closely resemble those of the largest pachyderms, as to suggest the idea, that unlike the rest of its class, the Iguanodon had the body supported as in the mammalia, and the abdomen suspended higher from the ground than in any existing saurians.

In fine, we have in the Iguanodon the type of the terrestrial herbivora, which in that remote epoch of the earth's physical history, termed by geologists "The Age of Reptiles," occupied the same relative station in the scale of being, and fulfilled the same general purposes in the economy of nature, as the Mastodons, Mammoths, and Mylodons, of the tertiary periods, and the existing Pachyderms.

With regard to the probable magnitude of the individuals to which the largest bones in my collection belonged, a general estimate only can be formed, because the relative proportions of the limbs, head, and body, are still unknown; sooner or later an entire, or a considerable portion of the, skeleton of a young Iguanodon will be brought to light, and yield the information necessary to enable the palæontologist to ascertain the dimensions, and delineate the physiognomy, of the living original.

The size of the largest Iguanodon has been estimated as follows: length of the head, three feet-of the trunk, twelve feet of the tail, thirteen feet-total length, twenty-eight feet.

This statement will surprise the reader who, from the popular notions of the magnitude of the Iguanodon, has entertained the idea that this reptile attained seventy feet in length; but the discrepancy between the above estimate and that first suggested by me, admits of an easy explanation.

In my earliest notices of the Iguanodon, which were published from time to time, as fresh discoveries disclosed new modifications of structure in this prodigious creature, an attempt was made to estimate the probable magnitude of the original, by instituting a comparison between the fossil bones and those of the Iguana; the recent type which the form of the teeth seemed to point out as the one most nearly related to the extinct reptile. It was thus shown that if the proportions were the same in both, the Iguanodon must have attained seventy feet or more in length. But this statement was qualified (more than eleven years since) by the remark"It is not, of course, pretended that such an estimate can offer more than a very distant approximation to the truth; yet it may be confidently affirmed that a reptile, which required a thigh-bone larger than that of the Elephant to support it, could not be of less colossal dimensions. In truth, I believe that its magnitude is here underrated, for, like Frankenstein, I was struck with astonishment at the monster which my investigations had, as it were, called into existence, and was more anxious to reduce its proportions than to exaggerate them. Should subsequent discoveries prove that the Iguanodon more nearly corresponded in the proportions of the tail with the Crocodilian family than with the Lizards, its total length would be much less than is here inferred; and from the shape of some of the metacarpals and phalangeals, it seems highly probable that the original was more bulky in proportion to its length, than the existing Lacertians."

In subsequent notices this opinion was reiterated, and on the discovery of several perfect anterior caudals, I expressed my conviction that the tail of the Iguanodon was shorter than in the Iguana, and instead of being long and round, was

"Brit. Rep." 1841, p. 144.

compressed laterally, and largely developed in a vertical direction. In my Memoir, in the "Phil. Trans." 1841 (pp. 137-140), it is stated that "from the shortness of the caudal vertebræ, and the length of the spinous processes, indicating a great vertical development of the tail, it is probable this organ was not long and slender, as in the Iguana, but approximated more nearly to the tail of the Doryphorus."

"The length of the united head and trunk, according to my estimate,1 is seventeen feet and a half; by Professor Owen's it is reduced to fifteen feet: a difference of no importance in such merely approximative calculations, particularly when the form of the cranium is unknown. The great discrepancy is in the estimated length of the tail; if the Iguanodon resembled the Iguana in its caudal proportions, its total length would be seventy feet; but if the tail was short, the total length of the animal would, of course, be proportionately reduced, and the most gigantic individuals may not have exceeded thirty feet in length."

A recent discovery, however, supports the idea first suggested by the stupendous size of the bones of the extremities.

In a block of calciferous grit picked up on the sea-shore, I have laid bare a chain of eleven caudal vertebræ, belonging to the middle region of the tail; and the bodies of these bones, instead of being abbreviated, as the shortness of the known anterior caudals led us to infer, are elongated as in the corresponding part of the skeleton of the recent Iguana. The length of four of these vertebræ is equal to that of five dorsals; and their spinous and transverse processes are so well developed, as to show that the tail must have been greatly prolonged-probably, in the same degree as in the existing lizards. The length of the femur of this individual is equal to six caudal, or eight anterior dorsal vertebræ. It is, therefore, according to the present state of our knowledge, not at all improbable, that the largest Iguanodons may have attained a length of from sixty to seventy feet.

Although some important points in the osteology of the Iguanodon are still unknown, we may safely conclude that this stupendous reptile equalled in bulk the large herbivo

1 66 Geology of the South-East of England," p. 316.