4OO MANUAL OF ZOOLOGY. 



which roughly correspond with the bony plates below, and 

 which constitute in some species the " tortoise-shell " of com- 

 merce. These epidermic plates, however, must on no account 

 be confounded with the true bony box in which the animal is 

 enclosed, and which is produced partly by the true endo- 

 skeleton, and partly by dermal integumentary ossifications. 



The other points of importance as regards the endoskeleton 

 are these : 



Firstly, The dorsal vertebrae are immovably joined together, 

 and have no transverse processes, the heads of the ribs uniting 

 directly with the bodies of the vertebrae. 



Secondly, The scapular and pelvic arches, supporting the 

 fore and hind limbs respectively (fig. 155, s and/), are placed 

 within the carapace, so that the scapular arch is thus inside the 

 ribs, instead of being outside, as it normally is. The scapular 

 arch consists of the shoulder-blade or scapula, and two other 

 bones, of which one corresponds with the acromion process of 

 human anatomy, and the other to the coracoid process, or to 

 the " coracoid bone " of the Birds. The clavicles, as is also 

 the case with the Crocodilia, are absent. 



The order Chelonia is conveniently divided into three sec- 



Fig. 156. Hawk's-bill Turtle (CJuIonia. imbricata}-^^ BelL 



tions, according as the limbs are natatory, amphibious, or ter- 

 restrial. In the first of these, the limbs are converted into 

 most efficient swimming-paddles, all the toes being united by a 

 common covering of integument. In this section are the well- 



