CHELONIAN REPTILES. 473 



feet. The impulse which they give being lateral 

 and oblique renders them more efficacious for 

 progression in the water than on land : this 

 circumstance, in conjunction with the constitu- 

 tional torpor of the animal, sufficiently accounts 

 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 

 skeleton. The ai'ticulations 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 brought together, and the whole limb withdrawn 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 exceedingly flattened, and ap- 

 proximate 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. The curvature of the humerus is of 

 great advantage to the tortoise in assisting it to 

 turn itself, when, by any accident, it has been 

 laid on its back. 



Considerable differences may be noticed in 

 tlie structure of the several species of Chelonia, 



