INTRODUCTION. IX 



important portions of their organization. If, with Cuvier, 

 and most other Zoologists, we include the amphibious 

 group in this class, these discrepancies are still more re- 

 markable ; but even restricting our view to the Reptilia 

 proper, they are sufficiently striking ; and a slight glance 

 at the general structure of two of the orders will exhibit 

 them in a very obvious point of view. 



In the Chelonians, or Tortoises, and in the Ophidians, 

 or Serpent tribe, the extremes of these different types of 

 organization are exhibited. In the common European 

 land Tortoise, Testudo Graca, which may be selected as 

 a familiar example of the former group, the whole struc- 

 ture of the skeleton is brought into the most compact 

 and solid state. The bones of the cranium and face are 

 consolidated into a single and immovable case, with 

 scarcely the vestige of sutures showing the separation of 

 the different centres of ossification upon which it has 

 been formed ; there are no teeth, but the margins of 

 the upper and of the lower jaw are covered by a horny 

 beak, the latter being received into a groove of the former, 

 and thus closing like the lid of a box ; then the whole 

 of the dorsal vertebrae, the ribs, the bones representing 

 the sterno-costal cartilages, and the broad united sternum, 

 are altogether compacted into a case of bone, without any 

 separation between the parts of which it is composed. 

 The anterior and posterior extremities are fully developed, 

 but, instead of being placed exterior to the thorax, they 

 are all of them contained within its cavity, and even the 

 bones of the feet are only extended beyond the horny box 

 which protects them, when the animal is employing them 

 in progression. 



What a contrast to this solid and compact structure 

 is exhibited by the form of the lithe and tortuous Ser- 



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