344 ZOOLOGY 



SECT. 



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. 983). 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, 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. Intercentra are present 

 throughout the spinal column in Sphenodon, and these are 

 represented in the Crocodilia by cartilaginous rings or discs the 

 intervertebral discs (Fig. 985 IS) : in the other orders the inter- 

 centra are of more irregular occurrence, but in the Lacertilia (as 

 well as in Sphenodon and the Crocodilia) there are a series of sub- 

 vertebral chevron bones in the caudal region. 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 Sphenodon, the Chelonia and Crocodilia have caudal ribs 

 which become fused with the bodies of the vertebrae. In the 

 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 Sphenodon (Fig. 984) and 

 Crocodilia (Fig. 983) each rib has connected with it posteriorly 

 a flattened curved cartilage, the uncinute. 



In the Chelonia (Fig. 986) the total number of vertebras 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 vertebras of the trunk, usually 

 ten in number, are immovably united with one another by means 

 of fibro-cartilaginous intervertebral discs. Each of the neural 

 spines, from the second to the ninth inclusively, is flattened and 

 fused with a flat plate of dermal origin the neural plate (Fig. 

 987, V\ and the row of plates thus formed constitutes 

 the median portion of the carapace. The ribs (R) are likewise 

 immovable ; a short distance from its origin each passes into a 

 large bony dermal costal plate ((?), and the series of costal plates 

 uniting by their edges form a large part of the carapace on either 

 side of the row of neural plates. The carapace is niade up of the 

 neural and costal plates supplemented by a row of marginal plates 

 (Figs. 986 and 987, M) running along the edge, and nuchal (Nu) 

 and pyyal (Py) plates situated respectively in front of and behind 



