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 wornout 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

and here we find a provision of another kind to give efficacy and strength. The front was traversed longitudinally by alternate ridges and furrows, (Pl. 24, Figs. 2, 5, 6, 7, 8), the ridges serving as ribs or 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. (Fig. 10, 11.)

* In Pl. 24, 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.

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.

SECTION XI.

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 sur

vived 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

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.

fishes, we might, à 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.*

During these grand periods of lacustrine mammalia, in which but few of the present genera of terrestrial carnivora had been called into existence, the important office of controlling the excessive increase of the aquatic herbivora appears to have been consigned to the Crocodiles, whose habits fitted them, in a peculiar degree, for such a service. Thus, the past history of the Crocodilean tribe presents another example of the well regulated workings of a

One of these, found by Mr. Spencer in the London clay of the Isle of Sheppy, is engraved, Pl. 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.

consistent plan in the economy of animated nature, under which each individual, whilst following its own instinct, and pursuing its own good, is instrumental in promoting the general welfare of the whole family of its cotemporaries.

Cuvier observes, that the presence of Crocodilean reptiles, which are usually inhabitants of fresh water, in various beds, loaded with the remains of other reptiles and shells that are decidedly marine, and the further fact of their being, in many cases, accompanied by freshwater Tortoises, shows that there must have existed dry land, watered by rivers, in the early periods when these strata were deposited, and long before the formation of the lacustrine tertiary strata of the neighbourhood of Paris.* The living species of the Crocodile family are twelve in number, namely, one Gavial, eight true Crocodiles, and three Alligators. There are also many fossil species: no less than six of these have been made out by Cuvier, and several

* M. Geoffroy St. Hilaire has arranged the fossil Saurians with long and narrow beaks, like that of the Gavial, under the two new genera, Teleosaurus and Steneosaurus. In the Teleosaurus, (Pl. 25'. Fig. 2.) the nostrils form almost a vertical section of the anterior extremity of the beak; in the Steneosaurus, (Pl. 25', Fig. 3.) this anterior termination of the nasal canal had nearly the same arrangement as in the Gavial, opening upwards, and being almost semi-circular on each side.-Recherches sur les grands Sauriens, 1831.