374 REPTILES xiv. 5 



significant features, characteristic of the reptiles. The head is carried 

 off the ground, on a well-developed neck. The two first cervical 

 vertebrae are modified to form the atlas and axis. The atlas is a ring 

 of bone without centrum, but with a facet in front for the occipital 

 condyle and one behind for the odontoid process, a peg attached to 

 the front of the axis but derived in development from the centrum of 

 the atlas segment. 



-sc. 



Fig. 212. Shoulder girdle and sternum of a lizard (Iguana). 



el. clavicle; cor. coracoid; gl. glenoid; int.cl. interclavicle; pr. procoracoid 

 process; sc. scapula; st.r. sternal rib; St. sternum. (From Reynolds, after Parker.) 



The vertebrae articulate with each other by a system of interlocking 

 processes much more elaborate than that found in fish-like vertebrates 

 and presumably serving to allow the column to carry weight. As a 

 rule, each centrum is concave in front, covering the convex hind end 

 of the vertebra next to it, a condition known as procoelous. In aquatic 

 vertebrates the centra articulate by flat surfaces and this condition was 

 retained in the amphicoelous vertebrae of many primitive reptiles. 

 Besides this articulation of the centra the vertebrae are also united by 

 the zygapophyses, facets on the neural arches, so arranged that the 

 upwardly facing surfaces of the anterior zygapophyses slide over the 

 down-facing surfaces of the posterior zygapophyses, an arrangement 

 that is found throughout the amniotes. 



Ribs are well developed in the middle or trunk region; each arti- 

 culates with the body of the vertebra by a single capitular facet. 



