SKULL. 357 



neural arch of the preceding vertebra. The neural arches are 

 completely fused with the centra. The transverse processes in 

 the trunk are short and carry the ribs. Some of the anterior 

 trunk vertebrae have hypapophyses. In Dasypeltis the an- 

 terior of these are tipped with an enamel-like substance (p. 368). 

 There are no chevron bones, but the transverse processes of the 

 caudal vertebrae have strong descending processes which have 

 the same relation to the caudal vessels. The atlas and axis are 

 similar to those of lizards. The vertebral column is capable 

 of a considerable amount of lateral flexion, but of very little, 

 if any, dorso- ventral flexion. 



The ribs are articulated to the transverse processes of the trunk 



FIG. 196. Anterior and posterior view of a trunk vertebra of a Python (from Huxley), zs 

 zygosphene ; z a zygantrum ; p z prezygapophysis ; pt z postzygapophysis ; t p transverse 

 process. 



vertebrae. They have capitula only, and are very moveable in 

 an antero-posterior direction. They are usually hollow and end 

 ventrally in a cartilage which is attached to the connective tissue 

 underlying the ventral shields. The movements of the ribs in 

 combination with the flexion of the vertebral column are of great 

 use in locomotion. Snakes run, in a sense, on the extreme 

 points of their ribs which are moved forwards, carrying with 

 them the ventral shields to which they are attached. There is 

 no trace of a sternum in any snake. 



The skull presents a number of peculiarities in which it differs 

 from the ordinary lacertilian type ; but it is interesting to notice 

 that some of these crop up in different families of lizards. The 

 most important points of difference are the following. (1) The 

 cranial cavity is long and extends, but little diminished in depth, 

 into the interorbital region, there being no interorbital septum. 



