CHELONIAN REPTILES. 469 



gument. It is this substance which is commonly 

 known by the name of tortoise shell.* 



The immobility of the trunk is compensated, 

 as far as regards the safety of the head, by the 

 great flexibility of the neck ; which is composed 

 of seven vertebrae, unencumbered by processes, 

 and capable of taking a double curvature like 

 the letter S, when the head is to be retracted 

 within the carapace. These vertebrae are joined 

 by the ball and socket articulation common to 

 all the existing species of reptiles. f The articu- 

 lation of the head with the neck is effected in 

 the same manner ; but it is interesting to remark 

 that the occipital condyle, which is situated at 

 the lower margin of the great aperture, though 

 presenting a single convex surface, yet has that 



surface evidently divided 

 into three parts ; the two 

 upper portions being late- 

 JJ ral, and the lower portion in 

 the middle. These three 

 articular surfaces are seen 

 immediately below the central aperture, F, in Fig. 



* It should be observed, that the divisions of these plates, 

 which appear externally, bear no relation to the sutures which 

 separate the subjacent bones, so that it is not possible to draw 

 inferences respecting the form of the latter from the mere inspec- 

 tion of the external shell. 



f The expression of this fact is thus qualified, because it does 

 not apply to many fossil or extinct species, such as the Ichthyo- 

 saurus. 



