THE CHELONIA. li'' 



account of ttese systems in Aves. But modifications in the 

 myology, neurology, splanchology, and development of tlie 

 two classes may be conveniently considered together, under 

 those several heads, in another chapter. 



I. The Chelonia. — The Tortoises and Turtles are those 

 reptiles which most nearly approach the Amphibia, though 

 they depart very widely not merely from the amphibian, but 

 from the ordinary vertebrate type, in some respects. 



A horny epidermic exoskeleton is absent in the soft Tor- 

 toises [Trionyx), the bodies of which are covered by a soft 

 integument; but, in the other Chelonia, the epidermis is modi- 

 fied into horny plates, which constitute the so-called " Tor- 

 toiseshell," and have in general a very definite arrangement. 

 The dorsal surface of the body presents three series of 

 central plates, of which five are in the middle and four upon 

 each side (4, 5, 4). The margins of the dorsal shield are 

 guarded by twenty-four or twenty-five plates : one on the 

 middle line in front, called nuchal ; one or two behind. 2njcja} ; 

 and eleven on each side, marginal. The ventral shield some- 

 times presents an anterior median scale ; but, more usually, 

 there are six pairs, disposed symmetrically. It will be seen, 

 presently, that these epidermic plates by no means coii-e- 

 spond with the bony dermal ossifications. In addition to 

 these principal plates, smaller scale-like patches of horny 

 epidermis are developed on other parts of the body, and on 

 the limbs. 



The dermal ossifications may best be described in con- 

 nection with the endoskeleton. 



The presacral vertebrae are few in number. In the 

 Green Turtle {Chelone midas) there are eight cervical, and 

 ten dorsal, in front of the sacrum, which is composed of 

 two vertebrae. In aU the cervical vertebrae the neurocentral 

 sutures persist ; there are no transverse processes, or libs, 

 and the spines are low or obsolete. The first vertebra, or 

 atlas, is a ringlike bone, composed of three pieces, one 

 basal and two superolateral. The second is a true axis 

 vertebra, the central part of the centrum of the atlas ossify- 



