FOSSIL TESTUDINATA. 195 



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 de- 

 velopement. The first appearance of all these three fami- 

 lies 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 Ple^ 

 siosauri, became extinct, and forms of Crocodiles, approach- 

 ing 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, Avhose 

 movements are usually slow and torpid, by enclosing the 

 body within a double shield or cuirass, formed by the ex- 

 pansion of the vertebra), ribs and sternum, into a broad 

 bony case. 



The small European Tortoise, Testudo Graeca, and the 

 eatable Turtle, Chelonia Mydas, are familiar examples of 

 this pecuUar 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 



