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 development. 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 in closing the body within a double shield or cuirass, formed by the expansion of the vertebrae, 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
