544 



REPTILIA. 



serpent, all four extremities first make their appearance externally. 

 As the legs become increased in their relative size and importance, 



Fig. 243. 



the trunk is proportionately shortened and its flexibility diminished 

 (Jig. 243), until at length we are conducted almost by impercep- 

 tible gradations to the strong and voracious Crocodiles, the most 

 perfect of the Reptile families. 



(592.) The fourth order of Reptiles (CHELONIA) comprises a 

 series of animals of most anomalous conformation, in which the 

 greater part of the skeleton is brought quite to the exterior of the 

 body, and the limbs are absolutely enclosed within the cavity form- 

 ed by the ribs. Such are the Tortoises and the Turtles (Jig. 244) ; 

 but, as we shall describe the anatomy of these animals more at 

 length hereafter, we need only in this place point out to the reader 

 their outward form and general appearance. 



(593.) Commencing our researches concerning the internal organ- 

 ization of this extensive class by examining the osteology of the 

 Reptilia, we shall, as we have hitherto done, select one skeleton for 

 special examination ; and afterwards, taking that as a standard of 

 comparison, observe the most conspicuous modifications of struc- 

 ture met with in the different divisions of this important group. 



