xni 



PHYLUM CHORDATA 



333 



Intercentra may be represented by intervertebral discs of fibro- 

 cartilage (Crocodilia) or by bony elements formed by ossification of 

 the ventral portions of the discs (Geckos, Sphenodon). In Lizards 

 in general and the Crocodiles there are inferior processes (hypapo- 

 physes], perhaps representing intercentra, situated below the centra 

 in the anterior cervical region. Chevron bones (inferior arches) 

 occur in the caudal region of many Reptiles (Sphenodon, Lacertilia, 

 Crocodilia). 



In the Snakes and in Iguanas, in addition to the ordinary 

 articulating processes or zygapophyses, there are peculiar articular 

 surfaces termed zygosphenes and zygantra (Fig. 993). The zygo- 

 sphene is a wedge-like process projecting forwards from the anterior 

 face of the neural arch of the vertebra, and fitting, when the vertebrae 

 are in their natural positions, into a depression of corresponding 



II. S. 



/it. 5. 



FIG. 992. Vertebra 

 of Sphenodon, 



showing the amphi- 

 coelous centrum (C.). 

 (After Headley.) 



FIG. 993. Vertebra of Python, anterior and posterior views, n. s. 

 neural spine ; p. z. prezygapophyses ;] pt. z. post-zygapophysis : 

 t. -p. transverse processes; z. [a. zygantrum ; z.s. zygosphene. 

 (After Huxley.) 



form the zygantrum on the posterior face of the neural arch 

 of the vertebra in front. To this arrangement, as well as to the 

 deeply concavo-convex centra, the extraordinary flexibility and 

 strength of a Snake's back-bone are due. 



The various regions of the spinal column are well marked in 

 most of the Lizards, in the Rhynchocephalia, in the Chelonia, and 

 in the Crocodilia (Fig. 994). In the Snakes and many of the 

 snake-like Lizards only two regions are distinguishable pre-caudal 

 and caudal. In the others there is a sacral region comprising 

 usually two vertebrae, both of which have strong processes (sacral 

 ribs) for articulation with the ilia. The first and second vertebrae 

 are always modified to form an atlas and axis : in the Lacertilia 

 and Chelonia the latter has a distinct odontoid process. In 

 Chamaeleons, Sphenodon, and the Crocodiles there is a median bone, 

 the pro-atlas (Fig. 996, 0), intercalated between the atlas and the 

 occipital region of the skull. 



Ribs are developed in connection with all the vertebrae of the 

 pre-sacral or pre-caudal region ; in the caudal region they are 



