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. 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 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 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 * 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. 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 Pro vence. 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. 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 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 contemporaries. 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 farther fact of their being, in many cases, accompanied by fresh-water 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 Giaval, 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 others, from the secondary and tertiary formations in England remain to be described.† * 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. † One of the finest specimens of fossil Teleosauri yet discovered, (see Pl. 25, Fig. 1,) was found in the year 1824, in the alum shale of the lias formation at Saltwick, near Whitby, and is engraved in Young and Bird's Geological Survey of the Yorkshire Coast, 2d Ed. 1828: its entire length is about eighteen feet, the breadth of the head twelve inches, the snout It would be foreign to our present purpose, to enter into a minute comparison of the osteology of living and fossil genera and species of this family. We may simply observe, with respect to their similar manner of dentition, that they all present the same examples of provision for extraordinary expenditure of teeth, by an unusually abundant store of these most essential organs.* As Crocodiles increase to no less than four hundred times their original bulk, between the period at which they leave the egg and their full maturity, they are provided with a more frequent succession of teeth than the mammalia, in order to maintain a duly proportioned supply during every period of their life. As the predaceous habits of these animals cause their teeth, placed in so long a jaw, to be peculiarly liable to destruction, the same provision serves also to renew the losses which must often be occasioned by accidental fracture. The existence of these remedial forces, thus uniformly adapted to supply anticipated wants, and to repair foreseen injuries, affords an example of those supplementary contrivances, which give double strength to the argument from design, in proof of the agency of Intelligence, in the construction and renovation of the animal machinery in which such contrivances are introduced. The discovery of Crocodilean forms so nearly allied to was long and slender, as in the Gavial, the teeth, one hundred and forty in number, are all small and slender, and placed in nearly a straight line. The heads of two other individuals of the same species, found near Whitby, are represented in the same plate, Figs. 2, 3. Some of the ungual phalanges, which are preserved on the hind feet of this animal, Fig. 1, show that these extremities were terminated by long and sharp claws, adapted for motion upon land, from which we may infer that the animal was not exclusively marine; from the nature of the shells with which they are associated, in the lias and oolite formations, it is probable that both the Steneosaurus and Teleosaurus frequented shallow seas. Mr. Lyell states that the larger Alligator of the Ganges, sometimes descends beyond the brackish water of the delta into the sea. * This mode of dentition has been already exemplified in speaking of the dentition of the Ichthyosaurus, P. 136, and Pl. 11. A. the living Gavial, in the same early strata that contain the first traces of the Ichthyosaurus and the Plesiosaurus, is a fact which seems wholly at variance with every theory that would derive the race of Crocodiles from Ichthyosauri and Plesiosauri, by any process of gradual transmutation or developement. The first appearance of all these three families of reptiles seems to have been nearly simultaneous; and they all continued to exist together until the termination of the secondary formations; when the Ichthyosauri and Plesiosauri, became extinct, and forms of Crocodiles, approaching to the Cayman and the Alligator, were for the first time introduced. SECTION XII. FOSSIL TORTOISES, OR TESTUDINATA. AMONG the existing animal population of the warmer regions of the earth, there is an extensive order of reptiles, comprehended by Cuvier under the name of Chelonians, or Tortoises. These are subdivided into four distinct families; one inhabiting salt water, two others fresh-water lakes and rivers, and a fourth living entirely upon the land. One of the most striking characters of this Order consists in the provision that is made for the defence of creatures, whose movements are usually slow and torpid, by enclosing the body within a double shield or cuirass, formed by the expansion of the vertebræ, ribs and sternum, into a broad bony case. The small European Tortoise, Testudo Græca, and the eatable Turtle, Chelonia Mydas, are familiar examples of this peculiar arrangement both in terrestrial and aquatic reptiles; in each case the shield affords compensation for the want of rapidity of motion to animals that have no |