320 ZOOLOGY SECT. 



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 backbone are due. 



The various regions of the spinal column are well marked in 

 most of the Lizards, in the Chelonia and in the Crocodilia (Fig. 

 927). 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 two vertebrae, all of which 

 have strong transverse processes for articulation with the ilia. The 

 first and second vertebrae are always modified to form an atlas and 

 axis. Ribs are developed in connection with all the vertebrae of" 

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

 usually replaced by inferior arches ; but the Chelonia have caudal 

 ribs sometimes fused with the bodies of the vertebrae. In the 



FIG. 926. Vertebra of Python, anterior and posterior views, n. s. neural spine ; pt. 2. post r, 

 p. z. prezygapophyses ; t. p. transverse processes ; z. a. zygantrum ; zs. zygosphene. (After- 

 Huxley.) 



Lacertilia only a small number (three or four) of the most anterior 

 of the thoracic ribs are connected with the sternum by cartila- 

 ginous sternal ribs ; the rest are free, or are connected together into 

 continuous hoops across the middle line. In the so-called Flying 

 Lizards (Draco) a number of the ribs are greatly produced, and 

 support a pair of wide flaps of skin at the sides of the body, acting 

 as wings, or rather as parachutes. In Hatteria (Fig. 928) and 

 Crocodilia (Fig. 927) each rib has connected with it posteriorly 

 a flattened curved cartilage, the uncinate. In the Crocodilia 

 (Fig. 929) there are intercalated between the centra a series of 

 cartilaginous discs, the intervertebral discs (IS) ; only three or four 

 ribs are connected with the sternum. 



In the Chelonia (Fig. 930) the total number of vertebrae is 

 always smaller than in the members of the other orders. The 

 cervical ribs are small and fused with the vertebrae. The cervical 

 and the caudal are the only regions in which the vertebrae are 

 movable upon one another. The vertebrae of the trunk, usually 



