470 THE MECHANICAL FUNCTIONS. 



215,which exhibits the skull of the Testudo my das, 

 viewed from behind. Although closely approxi- 

 mated, a faint line of demarcation, which divides 

 their surface, indicates an incipient tendency to 

 separate ; we shall find that in the further steps 

 of developement which occur in the higher classes, 

 this separation actually takes place by the obli- 

 teration of the lower articular surface, and the 

 transfer of the two lateral surfaces to the con- 

 dyloid processes arising from the developement 

 of the leaves of the occipital bone. 



The singular conformation of the bones of the 

 head in the turtle affords fresh evidence in sup- 

 port of the theory that these bones were origi- 

 nally vertebrae. The brain of the tortoise is 

 exceedingly small ; and yet the skull, when 

 viewed from above, presents an appearance of 

 great breadth, as if it enclosed a cavity of 

 large dimensions. But if we look upon it from 

 behind, as is shown in Fig. 21 5, we soon discover 

 that the real cavity in which the brain is lodged, 

 and to which the aperture at F leads, is very 

 small, only just admitting the end of the finger, 

 and that the broad plates of bone, P, P, which 

 form the upper surface of the skull, have no 

 relation to this cavity, and are merely extended 

 over the temporal muscles, which are of very 

 large size, occupying the whole of the spaces, 

 s, s ; which spaces are completely surrounded 

 by these bones. It would appear that the same 



