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, b\ 



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 bridges uniting 

 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 cehtrum 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. 



