426 MANUAL OF ZOOLOGY. 



ful. By some, the plastron is still regarded as a greatly-devel- 

 oped breast-bone or sternum. By others, again, the Chelonia 

 are regarded as being wholly without a sternum, and the 

 bones of the plastron are looked upon as exclusively integu- 

 mentary ossifications. Both the carapace and the plastron 

 are covered by a leathery skin, or more generally by a series 

 of horny plates (fig. 165), which roughly correspond with 

 the bony plates below, and which constitute in some species 

 the " tortoise-shell " of commerce. These epidermic plates, 

 however, must on no account be confounded with the true 

 bony box in which the animal is enclosed, and which is pro- 

 duced partly by the true endoskeleton 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 vertebras. 



Secondly, The scapular and pelvic arches, supporting the 

 fore and hind limbs respectively (fig. 164, 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- 

 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- 

 known 'YviX\\t%{Cheloniidce), all of which swim with great ease and 

 power, but are comparatively helpless upon the land (fig. 165). 

 The legs are of unequal length, and the carapace is much de- 

 pressed and flattened. The best-known species are the "edible" 

 or Green Turtle (Chelonia mydas), the Loggerhead Turtle 

 {Chelonia caouanna), the Hawk's-bill Turtle (C imbricatd), 

 and the Leathery Turtle [Sphargis coriaced). The Green Turtle 

 is largely imported into this country as a delicacy, and occurs 

 abundantly in various parts of the Atlantic and Indian Oceans. 

 The Hawk's-bill Turtle is of even greater commercial impor- 

 tance, as the horny epidermic plates of the carapace constitute 

 the "tortoise-shell" so largely used for ornamental purposes. 

 The Leathery Turtle is remarkable in having the carapace 



