276 THE VERTEBKATE SKELETON. 



Lacertilia is traversed by an unossified septum along which it 

 readily breaks. 



Chevron bones occur below the caudal vertebrae in Lacertilia, 

 Chelonia, Ichthyosauria, many Dinosauria, and Sphenodon, 

 articulating with quite the posterior part of the centrum which 

 bears them. In Lacertilia and Orocodilia (fig. 41, 3) the axis 

 has a well-marked odontoid process. The ventral portions of 

 the intervertebral discs are sometimes ossified, forming wedge- 

 shaped intercentra, as in Geckos, and the cervical vertebrae of 

 Sphenodon. 



In snakes, Theropod Dinosaurs, and the iguanas among 

 lizards, the neural arches are provided with zygosphenes, and 

 zygantra. 



The neural arches are usually firmly ankylosed to the 

 centra, but in the Crocodilia and some Chelonia, Sauropterygia, 

 and Dinosauria, the suture between the centrum and neural 

 arch persists at any rate till late in life. In the Ichthyosauria 

 the neural arches were united to the centra by cartilage only. 



The thoracic vertebrae of some of the Theromorpha (Dime- 

 trodon) are remarkable for the extraordinary development of 

 the neural spine, and those of Chelonia for the absence of 

 transverse processes. 



In living reptiles the number of sacral vertebrae is nearly 

 always two, but in the Theromorpha, Dinosauria, and Ptero- 

 sauria, as many as five or six bones may be ankylosed together 

 in the sacral region. In Crocodiles the two halves of the pelvis 

 sometimes articulate with different vertebrae. The vertebrae 

 of some of the great Sauropoda are remarkably hollowed out, 

 having a large vacuity on each side of the centrum communi- 

 cating with a series of internal cavities. The whole structure 

 of these vertebrae shows a combination of great strength with 

 lightness. 



THE SKULL. 



The reptilian skull is well ossified and the bones are noticeable 

 for their density. The true cranium is often largely concealed 



