CHELONIA. 45 I 



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. Of the nine pieces of the plastron, eight 

 are in pairs, whilst the ninth is odd, and is placed between the 

 four anterior ones. Both the carapace and the plastron 

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

 of horny plates (fig. 195), 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 endskeleton 

 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. 194, s and /), are placed 

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

 ribs, instead of being outside r 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 Turtles (Cheloniida), all of which swim with great ease 

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

 195). The legs are of unequal length, and the carapace is much 

 depressed and flattened. The best-known species are the 

 "edible" or Green Turtle (Chelone my das], the Loggerhead Tur- 

 tle ( Chelone caonanna), the Hawk's-bill Turtle (C. imbricata), 

 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 



