198 THE ANATOMY OF VERTEBRATED ANIMALS. 



ing apart, as an odontoid bone, and attaching itself to the 

 front face of the centrum of the second vertebra. 



The other cervical vertebrae are remarkable for the singular 

 variety in the disposition of their articular cups and balls. 



Fi.r (51. 



The Alligator Terrapeue {Chelydra Serpentina). 



Thvis, the third is opisthocoelous ; the fourth, biconvex : 

 the fifth, procoelous ; the sixth, also procoelous, but the 

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

 both the anterior and the posterior faces are very broad 

 and flattened, the posterior being the more convex. The 

 eighth cervical vertebra is procoelous, and differs from the 

 rest by the exj^ansiou of its neural spine, and by the 

 arching backwards of its postzygapophyses over the convex 

 prezygapophyses of the first dorsal vertebra, upon which 

 the former play backwai-ds and forwards. 



All the cervical vertebrae are very freely moveable upon 

 one anothei', and confer great flexibility on the neck. In 

 striking contrast with this an-angement, the ten following 

 vertebrae have flattened faces, firmly united by cartilage. 

 If any one of these vertebrae, from the second to the ninth, 

 be examined, it will be found that the elongated centrum is 

 only loosely united with the neural arch, and that the 

 simimit of the neural arch is continuous with a broad flat 

 plate of bone, which forms one of the eight median elements 

 of the carapace, or neural plates (Fig. 62, V). 



There are no transverse processes, but a rib is articulated 



