1 6 ANATOMY OF A CHELONIAN. 



is long and conspicuous, the portions of costal plate in 

 front of and posterior to it in that region being not yet 

 ossified (lo, />>). 



Also, examine more carefully the marginal plates 

 now that they are separated ; and note their relations 

 to the various costal plates. 



19. The Plastron is composed entirely of- dermic 

 bones. It consists of nine pieces : one median un- 

 paired bone toward its anterior end, called the ento- 

 PLASTRON ; and eight bones arranged in pairs on each 

 side of the median line ; those On each side, from be- 

 fore back, are named successively, epiplastron (? clav- 

 icle), HYOPLASTRON, HYPOPLASTRON, and XIPHIPLASTRON. 



The outward ends of the hyo- and hypo-plastron curve 

 dorsally, and make up most of the-bony bridgesuniting 

 carapace and plastron. 



20. The Cervical Vertebrae are eight in number and 

 freely movable, except the first and second. In gen- 

 eral, each consists of a centrum and neural arch, with 

 pre- and post-zygapophyses ; and the articular surfaces 

 of the prezygapophyses look dorsally, and those of 

 the postzygapophyses ventrally ; there are no trans- 

 verse processes, but the arches present more or less 

 of a median dorsal ridge, representing a rudimen- 

 tary spinous process. The posterior end of the body 

 is produced backward and ventrally some distance be- 

 yond the neural arch ; and the intervertebral foramina 

 are arched over by the articular processes. The centra 

 bear a slight median ridge on the ventral aspect, and 

 are thick and cylindrical near the anterior end, but 

 flattened dorso-ventrally at the posterior. 



