THE CHELONIA. 171 



on each side, 'marginal. The ventral shield sometimes pre- 

 sents 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 correspond with the 

 bony dermal ossifications. In addition to these principal 

 plates, smaller scale-like patches of horny epidermis are dc" 

 veloped on other parts of the body, and on the limbs. 



The dermal ossifications may best be described in connec- 

 tion with the endoskeleton. 



Tlie presacral vertebras are few in number. In the Green 

 Turtle ( Chelone midas) there are eight cervical, and ten dor- 

 sal, in front of the sacrum, which is composed of two vertebrae. 

 In all the cervical vertebrae the neurocentral sutures persist; 

 there are no transverse processes, or ribs, and the spines are 

 low or obsolete. The first vertebra, or atlas, is a ringlike 

 bone, composed of three pieces, one basal and twj superolat- 

 eral. The second is a true axis vertebra, the central part of 

 the centrum of the atlas ossifying apart, as an odontoid bone, 

 and attaching itself to the front face of the centrum of the sec- 

 ond vertebra. 



The other cervical vertebrae are remarkable for the singu- 

 lar variety in the disposition of their articular cups and balls. 



Fig. 61. — The Alligator Terrapene {Chelydra Serpentina). 



Thus the third is opisthocoelous ; the fourth, biconvex ; 

 the fifth, procoelous ; the sixth, also procoelous, but the . pos- 

 terior face is nearly flat, and very broad ; in the seventh, both 

 the anterior and the posterior faces are very broad and flat- 

 tened, the posterior being the more convex. The eighth cer- 

 vical vertebra is procoelous, and differs from the rest by the 

 expansion of its neural sj)ine, and by the arching backward 

 of its postzygapophyses over the convex prezygapophyses of 



