VERTEBRATA: REPTILIA. 535 



scutes ; and (3) a series of "marginal" scutes. The "verte- 

 bral " scutes, however, are only jive in number ; and each 

 series of "costal" scutes consists only of fotir pieces, so that 

 the number of epidermic plates is much smaller than that of 

 the bony plates beneath. The "marginal" scutes, on the 

 other hand, correspond in number with the " marginal plates " 

 beneath them. They are, therefore, twenty-four or twenty-six 

 in number, the anterior scute in the middle line being distin- 

 guished by the epithet of " nuchal," while the corresponding 

 scute behind is termed " pygal." 



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. 290, 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 pro- 

 cess, or to the " coracoid bone," of the Birds. The clavicles, 

 as is also the case with the Crocodilia, are absent ; but the 

 three anterior pieces of the plastron may represent an inter- 

 clavicle and clavicles. 



The order Chelonia is conveniently divided into three sections, according 

 as the limbs are natatory, amphibious, or terrestrial. 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. 294). The 

 legs are of unequal length, and the carapace is much depressed and flat- 

 tened. The best-known species are the " edible " or Green Turtle (Chelone 

 midas), the Loggerhead Turtle (Chelone caouanna), the Hawk's-bill Turtle 

 (C. imbricatd), and the Leathery Turtle (Sphargis coriacea). The Green 

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

 dantly in various parts of the Atlantic and Indian Oceans. The Hawk's- 

 bill Turtle is of even greater commercial importance, as the horny epi- 

 dermic plates of the carapace constitute the "tortoise-shell" so largely 

 used for ornamental purposes. The Leathery Turtle is remarkable in 

 having the carapace covered with a leathery skin in place of the horny 

 plates which are found in other species. 



In the second section of the Chelonia, in which the limbs are adapted for 

 an amphibious life, are the Mud-turtles or soft Tortoises ( Trionycidce), and 

 the Terrapins (Emydida;]. In the Trionyddtz the development of the 

 carapace is imperfect, the ribs being expanded and united to one another 

 only near their bases, and leaving apertures near their extremities. The 

 entire carapace is covered by a smooth leathery skin, and the horny jaws 



