Chap. IX. J 



VERTEBRA. 



are articulated with one another by means of pro- 

 cesses directed forwards and backwards, the zyga- 

 popliyses, as these articulating outgrowths are called. 



The shape of the faces of the centra of the ver- 

 tebrae varies greatly, not only in different forms of 

 the Saiiropsida, but even in different parts of the 

 vertebral column of the same individual. In the 

 Opliidia these differences are seen to be associated 

 with their mode of life ; the anterior face is deeply 

 hollowed, and the posterior 

 rounded and convex ; the 

 convexity fitting into the 

 concavity of the next suc- 

 ceeding vertebra, and being 

 capable of rotation within it ; 

 in addition to this, the faces 

 of the neural arch are modi- 

 fied, the anterior being pro- 

 duced into two wedge-shaped 

 processes (zygosplieiies), 

 which fit into corresponding 

 depressions (zygaiitra) on 

 the hinder face of the arch, and 



thereby form a kind of peg-and-socket joint (Fig. 130). 

 Vertebrae of this kind are found also in the lacertiliaii 

 Iguanae, who are known to swim by the movements 

 of their tails. 



In Hatteria, the Geckos, and some fossil lizards, 

 the notochord is persistent between the vertebrae, 

 the centra of which are, therefore, amphicoelous. 

 Among the Crocodiles a progressive loss of the inter- 

 vertebral portion oi the notochord may be made out, 

 such forms as lived before the period of the Chalk 

 having amphicoelous, while cretaceous and post-cre- 

 taceous crocodiles have proccelous, vertebrae. Among 

 Birds, the fossil Archaeopteryx and Ichthyornis 

 appear to have had amphiccelous vertebrae, but in all 



Pig. 129. Section through the 

 Vertebra of a Frog, magni- 

 fied. 



ch, Notochord; cfis, its sheath; 

 o,c, different kinds of 

 (After Ecker.) 



