In instituting a comparison between the maxillary organs of the Iguanodon, and those of the existing herbivorous lizards, we are at once struck with their remarkable deviation from all known types in the class of reptiles. In the Amblyrhynchi (of the Galapagos Islands), 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, the lips not being more produced than in other reptiles; but this creature only bruises its food; it cannot grind or masticate it. In fact, the edentulous, expanded, scoop-shaped, procumbent symphysis of the lower jaw of the Iguanodon, has no parallel among either recent or fossil reptiles; and we seek in vain for organs at all analogous, except among the herbivorous mammalia. The nearest approach is to be found in certain Edentata; as for example in the Cholæpus didactylus, or Two-toed Sloth, in which the anterior part of the lower jaw is destitute of teeth, and much prolonged. The correspondence is still closer in the extinct gigantic Mylodons, in which the symphysis resembles the blade of a turf-spade, and has no traces of incisor sockets; and were not this part of the jaw elevated vertically in front, and the two sides confluent, it would present the very counterpart of that of the Iguanodon. The great number and size of the vascular foramina distributed along the outer side of the dentary bone in the Wealden reptile, and the magnitude of the anterior outlets which gave exit to the vessels 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; and there are strong reasons for supposing that the latter was greatly produced, and capable of being protruded and retracted so as to constitute, in conjunction with a long extensile tongue, a suitable instrument for seizing and cropping leaves and branches, which, from the construction of the teeth, we may infer was the food of the Iguanodon.
Thus we find the mechanism of the maxillary organs of the Wealden herbivorous saurian, as demonstrated by recent discoveries, in perfect harmony with the remarkable dental characters which rendered the first known teeth so enigmatical. In the Iguanodon 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 sloth-like Edentata of South America. In fine, we have in the Iguanodon the type of the terrestrial herbivora, which in that remote epoch of the earth's physical history—the Age of Reptiles—occupied the same relative station in the terrestrial fauna, and fulfilled the same general purposes in the economy of nature, as the Mylodons, Mastodons, and Mammoths, of the tertiary periods, and the large pachyderms of modern times.
Although some important data are still required to complete our knowledge of the structure of the Iguanodon, we are warranted in concluding that this colossal herbivorous reptile was as bulky as the elephant, and as massive in its proportions: for, living exclusively on vegetable substances, the abdominal region must have been largely developed. Its limbs must have been of proportionate size to support and move so enormous a carcass. The hinder extremities probably presented the unwieldly contour of those of the Hippopotamus, and were based on strong short feet, protected by broad horny ungueal phalanges, or nails. The fore-legs appear to have been less bulky, and adapted for seizing and pulling down plants and branches: the teeth and jaws demonstrate the nature of its food; and the fossil remains of coniferous trees, arborescent, ferns, and cycadeous plants, with which its relics are commonly associated, indicate the character of the flora adapted for its sustenance.[139]
[139] Philosophical Transactions, for 1848, pp. 196-198.
[XVIII.] The Pelorosaurus.—The humerus of a terrestrial reptile of enormous magnitude, has lately been discovered by Mr. Peter Fuller of Lewes, in the quarry near Cuckfield, from which many remains of the Iguanodon and Hylæosaurus were obtained in my early researches. This bone more nearly resembles the humerus of the Crocodiles, than that of the Lizards. It Is four and a half feet in length, and of corresponding proportions; it has a large medullary canal. As to the size of the animal to which it belonged, while disclaiming the idea that any certain conclusion can be drawn from a single bone, I may mention, with the view of conveying some general notion, that in a Gangetic crocodile eighteen feet long, the humerus is one foot: according to this scale the fossil animal would be eighty-one feet in length. I have proposed the name of Pelorosaurus (from πἑλωρ—pelòr—monster), or Colossal-saurian, for this new genus of reptiles which Inhabited the country of the Iguanodon.[140]
[140] A memoir on this fossil was read before the Royal Society, Feb. 14th, 1850; an abstract has been published in the Proceedings of the Society. It is entitled, "On the Pelorosaurus; an undescribed gigantic terrestrial reptile, whose remains are associated with those of the Iguanodon and other saurians in the strata of Tilgate Forest." It will appear in the Phil. Trans. Part 11. 1850.
[XIX.] Silicification, or petrifaction by flint.—The various forms in which silex occurs have depended on its state of fluidity. In quartz crystals the solution was complete; in agate and chalcedony it was in a gelatinous state, assuming a spheroidal or orbicular disposition according to the motion given to its particles. Its condition appears also to have been modified by the influence of organic matter. In some polished slices of siliceous nodules, the transition from flint to agate, chalcedony, and crystallized quartz, is beautifully shown. The curious fact that the shells of Echinites In chalk are almost invariably filled with flint, while their crustaceous shells are changed into calc-spar, is probably in many instances to be attributed to the animal matter having undergone silicification; for the most organized parts are those which appear to have been most susceptible of this transmutation. In some specimens the oyster is changed into flint, while the shell Is converted into crystallized carbonate of lime. In a trigonia from Tisbury, formerly In the cabinet of the late Miss Benett, of Norton House, near Warminster, the body of the mollusk was completely metamorphosed into pure chalcedony, the branchiæ or gills being as clearly defined as when the animal was recent. In specimens of wood from Australia (presented to the British Museum by Sir Thomas Mitchell), which are completely permeated by silex, there are on the external surface some spots of chalcedony, that have apparently originated from the exudation of the liquid silex from the interior in viscid globules filled with air, which burst, and then collapsed, and became solidified in their present form.
In silicified wood, the permeation of the vegetable tissues by the mineral matter appears to have been effected by solutions of silex of a high temperature. In some examples mineralization has been effected simply by replacement: the original substance has been removed, atom by atom, and the silex substituted in its place. One of the most eminent naturalists and mineralogists of the United States, Mr. J. D. Dana,[141] suggests that the reason why silica is so common a material in the constitution of fossil wood and shells, as well as in pseudo-morphic crystals,[142] consists in the ready solution of silex in water at a high temperature (a fact affirmed by Bergman[143]) under great pressure, whenever an alkali is present, as is seen at the present time in many volcanic regions, and its ready deposition again when the water cools. A mere heated aqueous solution of silica, under a high pressure, is sufficient to explain the phenomenon of the silicification of organic structures. Mr. Dana states that "a crystal of calc-spar in such a fluid, being exposed to solution from the action of the heated water alone, the silica deposits itself gradually on a reduction of temperature, and takes the place of the lime, atom by atom, as soon as set free. Every silicified fossil is an example of this pseudo-morphism; but there seems to be no union of the silica with the lime, for silicate of lime is of rare occurrence."[144]
[141] American Journal of Science, for January, 1845.