REPTILES 1399 



excessive and, indeed, proverbial tardiness of its 

 movements. 



Security appears still to be the object aimed at in 

 the mechanism of all other parts of the skeleton. 

 After the head has been drawn in by the double or 

 serpentine flexion of the neck, the knees are brought 

 together and the whole limb withdrawn within the 

 shell, the forelegs folding completely over the head, 

 so as to cover and protect it most effectually. 



Considerable differences may be noticed in the 

 structure of the several species of Chelonia, accord- 

 ing to the diversity of their habits. Tortoises which 

 live on land require more complete protection 

 by means of their shell than turtles, or Emydes, 

 which dwell only in the water: hence the convexity 

 of their carapace, the solidity of its ossification, its 

 immovable connection with the plastron, and the 

 complete shelter it affords to the head and limbs. 

 Turtles, on the other hand, receiving support from 

 the element in which they reside, require less pro- 

 vision to be made for these objects. Previously to 

 the retraction of the head and limbs within the shell, 

 the air is expelled from the large cavities of the 

 lungs by the vigorous actions of the abdominal 

 muscles, which exist in these animals as well as in all 

 the vertebrata, although here they are covered by 

 the bones, and compress the lungs by pushing the 

 abdominal ir iscera against them. This sudden ex- 

 pulsion of air is the cause of the long-continued hiss- 

 ing sound which the tortoise emits while preparing 

 to retreat into its stronghold. 



The ribs, though they first assume the form of 



F VOL. IV. 



