VERTEBRAL COLUMN. 



405 



number (often five or six) of inframarginals. In addition to 

 these scale-like plates, small homy structures are formed on 

 other parts of the body, especially on the limbs and head. 



The homy plates are formed as comifications of the outer parts of the 

 epidermis. They are added to throughout Ufe, increasing both in area 

 and thickness. The annual additions often leave marks in the form of 

 rings. 



The vertebrae are few in number. There are usually eight 

 cervical, ten trunk (thoracic), two sacral and a variable num- 

 ber of caudal. The cervical vertebrae are without transverse 

 processes or ribs, and the neural spines are low or absent. They 

 are freely moveable upon 

 one another, and their 

 neurocentral sutures persist. 



The trunk or thoracic 

 vertebrae bear ribs and are 

 firmly connected with the 

 carapace by their neural 

 spines and ribs They are 

 without transverse or articu- 

 lating processes, and the 

 neural arches are but loose- 

 ly attached to the centra. 

 The ribs are attached partly 

 to the neural arches and 

 partly to the centra. The 

 sacral vertebrae bear short 



ribs which are not ankylosed either to them or to the ilia. 

 The caudal vertebrae are procoelous and freely moveable. They 

 bear short ribs which may be fused with the vertebrae and 

 appear as transverse processes. 



The atlas is composed of three pieces, one ventral and two dorso-lateral. 

 The axis carries the centrum of the atlas as an odontoid process. The 

 character of the articulating surfaces of the centra varies considerably. 

 In Chelone midas the second and third vertebrae are opisthocoelous, the 

 fourth is biconvex, the fifth and sixth are procoelous, the seventh has a 

 flat anterior and a convex posterior surface, the eighth is concave in front 

 and convex behind. The centrum of the eighth is short, its neural spine 

 expanded (attached by ligament to the nuchal plate) ; its postzygapophyses 

 arch over the prezygapophyses of the first thoracic vertebra. The thoracic 

 vertebrae have flattened faces and are firmly united by cartUage. The 



Fig. 226. — The plastron of the green turtle 

 (Chelone midas) (from Huxley). I.cl inter- 

 clavicle ; d clavicles ; Hy.p hyoplastron ; 

 Hp.p hypoplastron ; Xp xiphiplastron. 



