others, from the secondary and tertiary formations in England remain to be described.* 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, * 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 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. 172, and Pl. 11. A. 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 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 ter mination 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 inclosing 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 ready means of escape by flight or concealment from their enemies. We learn from Geology that this Order began to exist nearly at the same time with the Order of Saurians, and has continued coextensively with them through the secondary and tertiary formations, unto the present time their fossil remains present also the same threefold divisions that exist among modern Testudinata, into groups respectively adapted to live in salt and fresh water, and upon the land. Animals of this Order have yet been found only in strata more recent than the carboniferous series.* The earliest example recorded by Cuvier, (Oss. Foss. Vol. 5, Pt. 2, p. 525), is that of a very large species of Sea Turtle, the shell of which was eight feet long, occurring in the Muschelkalk at Luneville. Another marine species has been found at Glaris, in slate referrible to the lower cretaceous formation. A third occurs in the upper cretaceous freestone at Maestricht. All these are associated with the remains of other animals that are marine; and though they differ both from living Turtles and from one another, they still exhibit such general accordance in The fragment from the Caithness slate, engraved in the Geol. Trans. Lond. V. iii. Pl. 16, Fig. 6, as portions of a trionyx, is pronounced by M. Agassiz to be part of a fish. the principles of their construction, with the conditions by which existing Turtles are fitted for their marine abode, that Cuvier was at once enabled to pronounce these fossil species to have been indubitably inhabitants of the sea.* The genera Trionyx and Emys, present their fossil species in the Wealden freshwater formations of the Secondary series; and still more abundantly in the Tertiary lacustrine deposits; all these appear to have lived and died, under circumstances analogous to those which attend their cognate species in the lakes and rivers of the present tropics. They have also been found : * Plate 25', Fig. 4, represents a Turtle from the slate of Glaris it is shewn to have been marine by the unequal elongation of the toes in the anterior paddle; because, in freshwater Tortoises, all the toes are nearly equal, and of moderate length; and in land Tortoises, they are also nearly equal, and short; but in all marine species they are very long, and the central toe of the anterior paddle, is by much the longest of all. The accordance with this latter condition in the specimen before us, is at once apparent; and both in this respect and in general structure, it approaches very nearly to living genera. This figure is copied from Vol. 5, Pt. 2, Tab. 14, f. 4, of the Oss. Foss. of Cuvier. M. Agassiz has favoured me with the following details respecting important parts which are imperfectly represented in the drawing from which Cuvier's engraving was taken. "The ribs show evidently that it is nearly connected with the genera Chelonia and Sphargis, but referrible to no known species; the fingers of the left fore paddle are five in number; the two exterior are the shortest, and have each three articulations; and the three internal fingers, of which the middle one is the longest, have each four articulations, as in the existing genera, Chelonia and Sphargis." |