* CHELONIAN REPTILES. ^25 



The immobilil}- of the trunk is compensated, as far as re- 

 gards 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 ca- 

 rapace. These vertebrae are joined by the ball and socket 

 articulation common to all the existing species of reptiles.* 

 The articulation of the head with the neck is effected in the 

 same manner; but it is interesting to remark that the occipi- 

 tal condyle, which is situated at the lower margin of the 

 great aperture, though presenting a single convex surface, 

 225 p^.^__,^__p yet has that surface evidently di- 



vided into three parts; the two up- 

 per portions being lateral, and the 

 lower portion in the middle. These 

 three articular surfaces are seen im- 

 mediately below the central aper- 

 ture, F, in Fig. 215, which exhi- 

 bits the skull of the Testudo my das, viewed from behind. 

 Although closely approximated, a faint line of demarcation, 

 which divides their surface, indicates an incipient tendency 

 to separate; we shall find that, in the farther steps of deve- 

 lopment which occur in the higher classes, this separation 

 actually takes place by the obliteration of the lower articu- 

 lar surface, and the transfer of the two lateral surfaces to the 

 condyloid processes arising from the development of the 

 leaves of the occipital bone. 



The singular conformation of the bones of the head, in the 

 turtle, affords fresh evidence in support of the theory that 

 these bones were originally vertebrae. The brain of the tor- 

 toise is exceedingly small; and, yet, the skull, when viewed 

 from above, presents an appearance of great brcadth^as if it 

 enclosed a cavity of large dimensions. But if we look upon 



so that it is not possible to draw inferences respecting the form of the latter 

 from the mere inspection of the external shell. 



* The expression of this fi\ct is thus qualified, because it does not apply 

 to many fossil or extinct species, such as the Iddhyosaurus. 



