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bone upon the nose, (PL 24, Fig. 14.) The concurrence of peculiarities so remarkable as the union of this nasal horn with a mode of dentition of which there is no example, except in the Iguanas, affords one of the many proofs of the universality of the laws of co-existence, which prevailed no less constantly throughout the extinct genera and species of the fossil world, than they do among the living members of the animal kingdom.
As the teeth are the most characteristic and important parts of the animal, I shall endeavour to extract from them evidence of design, both in their construction and mode of renewal, and also in their adaptation to the office of consuming vegetables, in a manner peculiar to themselves. They are not lodged in distinct sockets, like the teeth of Crocodiles, but fixed, as in Lizards, along the internal face of the dental bone, to which they adhere by one side of the bony substance of their root. (PI. 24, Fig. 13.)
The teeth of most herbivorous quadrupeds, (exclusively of the defensive tusks,) are divided into two classes of distinct office, viz. incisors and molars; the former destined to collect and sever vegetable substances from the ground, or from the parent plant; the latter to grind and masticate them on their way towards the stomach. The living Iguanas, which are in great part herbivorous, afford a striking exception to this economy: as their teeth are little fitted for grinding, they transmit their food very slightly comminuted into the stomach.
Our giant Iguanodon, also, had teeth resembling those of the Iguana, and of so herbivorous a character, that at first sight they were supposed by Cuvier to be the teeth of a Rhinoceros.
The examination of these teeth will lead us to the dis* covery of remarkable contrivances, adapting them to the function of cropping tough vegetable food, such as the Clathraria, and similar plants, which are found buried with the Iguanodon, might have afforded. We know the form and power of iron pincers to gripe and tear nails from their lodgment in wood: a still more powerful kind of pincers, or nippers, is constructed for the purpose of cutting wire, which yields to them nearly as readily as thread to a pair of scissors. Our figures (PI. 24, Figs. 6, 7, 8, 12) show the place of the cutting edges, and form of curviture, and points of enlargement and contraction, in the teeth of the Iguanodon, to be nearly the same as in the corresponding parts of these powerful metallic tools; and the mechanical advantages of such teeth, as instruments for tearing and cutting, must have been similar.*
The teeth exhibit also two kinds of provisions to maintain sharp edges along the cutting surface, from their first protrusion, until they were worn down to the very stump. The first of these is a sharp and serrated edge, extending on each side downwards, from the point to the broadest portion of the body of the tooth. (See Figs. 1, 2, 6, 8, 12, &c.)
The second provision is one of compensation for the gradual destruction of this serrated edge, by substituting a plate of thin enamel, to maintain a cutting power in the anterior portion of the tooth, until its entire substance was consumed in service.f
* Fig. 2. represents the front of a young tooth; and Figs. 5, 6,7,8 the front of four other teeth, thrown slightly into profile. In all of these we recognise a near approach to the form of the nipping pincers, with a sharp cutting edge at the upper margin of the enamel. The enamel is here expressed by wavy lines, which represent its actual structure: it is placed only in front, like the enamel in front of the incisors of Rodentia.
f This perpetual edge resulted from the enamel being placed only on the front of the tooth, like that on the incisors of Rodentia. As the softer material of the tooth itself must have worn away more readily than this enamel, and most readily at the part remotest from it, an oblique section of the crown was thus perpetually maintained with a sharp cutting edge in front, like that of the nippers. (See Figs. 7. 8.12.)
Whilst the crown of the tooth was thus gradually diminishing above, a simultaneous absorption of the root went on below, caused by the pressure of a new tooth rising to replace the old one, until by this continual consumption at both extremities, the middle portion of the older tooth was reduced to a hollow stump, (Figs,. 10,. 11,) which fell from the jaw to make room for a more efficient successor.* In this last stage the form of the tooth had entirely changed, and the crown had become flat, like the crown of worn out human incisors, and capable of performing imperfect mastication after the cutting powers had diminished. There is, I believe, no other example of teeth which possess the same mechanical advantages as instruments of cutting and tearing portions of vegetable matter from tough and rigid plants. In this curious piece of animal mechanism, we find a varied adjustment of all parts and proportions of the tooth, to the exercise of peculiar functions; attended by compensations adapted to shifting conditions of the instrument, during different stages of its consumption. And we must estimate the works of nature by a different standard from that which we apply to the productions of human art, if we can view such examples of mechanical contrivance, united with so much economy of expenditure, and with such anticipated adaptations to varying conditions in their application, without feeling a profound conviction that all this adjustment has resulted from design and high intelligence.
The younger tooth, (Fig. 1,) when first protruded, was lancet-shaped, with a serrated edge, extending on each side downwards, from the point to its broadest portion, as in the living Iguana. (PI. 24. / 13, and Fig. 4.) This serrature ceased at the broadest diameter of the tooth, i. e. precisely ul the line, below which, had they been continued, they would have had no effect in cutting. (PI. 24. f. 2. 6. 8. 9. -12 ) As these saws were gradually worn away, the cutting power was transferred to the enamel in front, and here we find a provision of another kind to give efficacy and strength. The front was traversed longitudinally by alternate ridges and furrows, (PI. 24, Figs. 2, 5, 6, 7, 8,) the ridges serving as ribs op buttresses to strengthen and prevent the enamel from scaling off, and forming, together with the furrows, an edge slightly wavy, and disposed in a series of minute gouges, or fluted chisels; hence the tooth became an instrument of greater power to cut tough vegetables under the action of the jaw, than if the enamel had been in a continuous straight line. By these contrivances, also it continued effective during every stage through which it passed, from the serrated lancet-point of the new tooth, (Fig. 1,) to its final consumption. (Figs. 10, 11.)
* In PI. 21, Fig. 13, the jaw of a recent Iguana exhibits the commencement of this process, and a number of young teeth are seen forcing their way upwards, and causing absorption at the base of the older teeth. Figs. 10, 11, exhibit the effect of similar absorption upon the residuary stump of the fossil tooth of an Iguanodon.
AMPHIBIOUS SAURIANS ALLIED TO CROCODILES.
The fossil reptiles of the Crocodilean family do not deviate sufficiently from living genera, to require any description of peculiar and discontinued contrivances, like those we have seen in the Ichthyosaurus, Plesiosaurus, and Pterodactyle; but their occurrence in a fossil state is of high importance, as it shows that whilst many forms of vertebrated animals have one after another been created, and become extinct, during the successive geological changes of the surface of our globe; there are others which have survived all these changes and revolutions, and still retain the leading features under which they first appeared upon our planet.
If we look to the state of the earth, and the character of its population, at the time when Crocodilean forms were first added to the number of its inhabitants, we find that the highest class of living beings were reptiles, and that the only other vertebrated animals which then existed were fishes; the carnivorous reptiles at this early period must therefore have fed chiefly upon them, and if in the existing family of Crocodiles there be any, that are in a peculiar degree piscivorous, their form is that we should expect to find in those most ancient fossil genera, whose chief supply of food must have been derived from fishes,
In the living sub-genera of the Crocodilean family, we see the elongated and slender beak of the Gavial of the Ganges, constructed to feed on fishes; whilst the shorter and stronger snout of the broad-nosed Crocodiles and Alligators gives them the power of seizing and devouring quadrupeds, that come to the banks of rivers in hot countries to drink. As there were scarcely any mammalia* during the secondary periods, whilst the waters were abundantly stored with fishes, we might ct priori, expect that if any Crocodilean forms had then existed they would most nearly have resembled the modern Gavial. And we have hitherto found only those genera which have elongated beaks, in formations anterior to, and including the chalk; whilst true Crocodiles, with a short and broad snout, like that of the Cayman and the Alligator, appear for the first time in strata of the tertiary periods, in which the remains of mammalia abound.f
During these grand periods of lacustrine mammalia, in which but few of the present genera of terrestrial carnivora
* The small Opossums in the oolite formation at Stonesfield, near Oxford, are the only land mammalia whose bones have been yet discovered in any strata more ancient than the tertiary.
t One of these, found by Mr. Spencer in the London clay of the Isle of Sheppy, is engraved, PI. 25', Fig. 1. Crocodiles of this kind have been found in the chalk of Meudon, in the plastic clay of Auteuil, in the London clay, in the gypsum of Mont Martre, and in the lignites of Provence.
The modern broad-nosed Crocodileans, though they have the power to capture mammalia, are not limited to this kind of prey; they feed largely also on fishes, and occasionally on birds. This omnivorous character of the existing Crocodilean family, seems adapted to the present general diffusion of more varied kinds of food, than existed when the only form of the beak in this family was fitted, like that of the Gavial, to feed chiefly on Fishes.