420 THE MECHANICAL FUNCTIONS. 



for the excessive, and indeed proverbial tardiness 

 of its movements. 



Security appears still to be the object aimed at 

 in the mechanism of all the other parts of the ske- 

 leton. The articulations at the shoulders and the 

 hips are such as facilitate the complete retraction 

 of the limbs within the carapace. 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 and the whole limb withdrawn within the shell, 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 fore legs folding completely over the head, so 

 as to cover and protect it most effectually. For 

 this purpose, the carpus and metacarpus are ex- 

 ceedingly flattened, and approximate to the fin- 

 like form, which we shall presently see exemplified 

 in the cetaceous tribes. The phalanges are also 

 large and lengthened, forming a kind of oval hand, 

 or rather paddle, the functions of which it is well 

 calculated to perform. Tiie curvature of the hu- 

 merus is of great advantage to the tortoise in assist- 

 ing it to turn itself, when, by any accident, it has 

 been laid on its back. 



Considerable diflerences 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 Emydcs, which 

 dwell in the water; hence the convexity of their 

 carapace, the solidity of its ossification, its im- 

 moveable connexion with the plastron, and the 

 complete shelter it aflbrds to the head and limbs. 

 Turtles, on the other hand, receiving support from 

 the element in which they reside, require less 

 provision to be made for these objects. Their 



