VERTEBRA AND RIBS 43 



cervicals are distinguished from the more posterior vertebrae by the 

 fact that the ribs, though present, do not extend to the sternum. 



The articulation of the vertebrae by zygapophyses is often 

 strengthened by a wedge-shaped process (zygosphene) on one face 

 of a centrum which fits in a corresponding cavity (zygantrum) on the 

 adjoining centrum (fig. 49) . A pecuharity of many reptiles, especially 

 lizards, is that the tail may break at a sHght strain, the line of fracture 

 passing through the middle of a vertebra which is weakened by the 

 persistent cartilage in the centrum ('glass snakes'). 



The history of a vertebra is well known in Sphenodon and Alligator and to a 

 less extent in some lizards. Sphenodon has the same formation of cranial and 

 caudal parts as in the lower groups, (p. 22), the cranials being the more promi- 

 nent, except in the tail where the two are subequal, the elements remaining 

 distinct for a time. Chondrification begins in the caudineurals and caudi- 

 haemals and extends up and down and also laterally in the myosepta, this last 

 part developing the transverse process and the rib. The successive neural arches 

 fuse above the spinal cord, and later 

 separate, with the resulting formation of 

 zygapophyses from the intermediate tissue. 

 Skeletogenous tissue (not cartilage) grows 

 inwards between the centra, forming inter- 

 vertebral discs, which, except in species with 

 permanently amphicoele vertebrae, restricts 

 and may even cut the notochord. On the 

 ventral side the caudal haemals are shifted 

 forwards to an intervertebral position and Fig. 50. — Five anterior vertebras 

 mav persist for a time at least in the trunk ^^ Sphenodon stage " S" (Howes and 



■ ^ . Swmnerton, oi). a, neural arch ot 



as intercentra, while in the tail they form atlas; ac, atlantean centrum; cr, 

 curved bars— hsmapophvses and haemal cervical ribs; d, dens epistrophei; e, 



, ,1 1 i, 1 • /i 1 1 centrum of epistropheus; h, hypo- 



arches, the latter bemg the chevron bones, chordalbar of atlas; «;., neurapophy- 



Separate centres of ossification occur in the sis; ns, neurocentral suture; nsp, 



centra and in the elements of the arches, and neural spme. 



for a time neural and haemal arches are 



connected with the centra by suture, a condition long persisting in Ichthyosaurs, 



many Crocodilia and some Chelonia. 



Squ.amata. — The number of vertebras in Squamata varies greatly, the largest 



number being found in apodal lizards (140) and in some snakes (400). In the 



older fossils and in geckos and Uropeltids the centra are usually amphicoelous; 



in other recent species they are procoelous. Intercentra occur in the necks of 



some Pythonomorphs, but are not known elsewhere. Zygosphenes and zygan- 



tra are developed in Ophidia, Iguana and some Pythonomorphs on the faces 



of the neural arches. Snakes have strong ventral processes (hypapophyses — 



fused hasmapophyses) on the anterior centra, and in Dasypeltis these protrude 



into the oesophagus and serve to saw the eggs on which these snakes feed. 



