56 
SYSTEM OF NATURE. 
shapeless fin. Nor is there, in the other organs of the 
body, any more considerable difference of development. 
But in the 'present class , the discrepancies are far more 
conspicuous , particularly in the whole constitution of the 
skeleton , in the organs of motion , in the integuments , and 
many other important portions of their organization. 
* * * * In the Chelonians or tortoises, and in the 
Ophidians or serpent tribe, the extremes of these different 
types of organization are exhibited. In the common Eu¬ 
ropean land-tortoise, Testudo Graeca, which may be selected 
as a familiar example of the former group, the whole struc¬ 
ture of the skeleton is brought into the most compact and 
solid state. The bones of the cranium and face are con¬ 
solidated into a single and immovable case, with scarcely 
the vestige of sutures, showing the separation of the dif¬ 
ferent centres of ossification 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 
vertebra?, the ribs, the bones representing the sterno-costal 
cartilages, and the broad united sternum, are altogether 
compacted into a case of bone, without any separation 
between the parts of which it is composed. The anterior 
and posterior extremities are fully developed, but instead 
* The skull, if that term be restricted to the bony covering of the 
brain, is quite invisible from the exterior, after the removal of the skin : 
the two temporal muscles occupy an immense space between the exterior 
bony covering of the head and the interior bony covering of the brain , 
which, in these animals, is remarkably small. The heads of a tortoise 
and a bird present us with the extreme differences of osteological 
structure. 
