INTRODUCTION. IX 



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

 Serpent tribe, the extremes of these different types of organ- 

 ization are exhibited. In the common European land Tor- 

 toise, Testudo Graca, which may be selected as a familiar 

 example of the former group, the whole structure 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 immoveable case, with scarcely the vestige of su- 

 tures showing the separation of the different centres of ossifi- 

 cation 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 sepa- 

 ration between the parts of which it is composed. The an- 

 terior and posterior extremities are fully developed, but in- 

 stead 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 pro- 

 gression. 



What a contrast to this solid and compact structure is 

 exhibited by the form of the lithe and tortuous Serpent ! 

 Most of the bones of the head are permanently separate ; 

 those of the upper and lower jaw particularly being capable of 

 great extension ; there are perfect teeth ; the vertebrae, which 

 are extremely numerous, are susceptible of the most extensive 

 lateral motion ; and the ribs, slender and but slightly at- 

 tached, compensate for the absence of both anterior and pos- 

 terior extremities, by being themselves the instruments of the 

 animal's progression. It is unnecessary here to enter more 

 particularly into the detail of these curious diversities of 



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