106 MORPHOLOGICAL TYPES 



Vertebral Column. — In Reptiles and all higher vertebrates, 

 the first two vertebrae are peculiarly modified. The first is 

 called the atlas, and its anterior surface is hollow to receive the 

 condyle of the skull. Its centrum has, however, been separated 

 from it and attached to that of the second vertebra, forming the 

 odontoid peg. Round this peg the atlas and skull are free to 

 rotate. The second vertebra is called the axis. The subse- 

 quent vertebrae of the neck and thorax are normal, and consist 

 of centra with neural arches and spines, and zygapophyses. 

 The centra are concave anteriorly, and convex posteriorly, a 

 condition described as proccelous. 



The vertebrae of the tail are peculiar in that they are split 

 transversely, and when the lizard sheds (autotomises) its tail 

 the break occurs at one of these splits. Under some of the 

 tail-vertebrae are Y-shaped haemal arches, ossified as " chevron- 

 bones." 



Ribs are carried by all vertebrae in front of the sacrum except 

 the first three ; they articulate with the centra of their 

 respective vertebrae. The ribs belonging to the vertebrae 

 of the neck (cervical) are short, those of the anterior region of 

 the thorax are attached ventrally to the sternum (five pairs). 

 The more posterior ribs (" floating ") do not touch the sternum. 

 The two sacral vertebrae bear stout transverse processes which 

 are attached to the ilia of the pelvic girdle. 



Pectoral Girdle and Limb. — The cartilage-bones of the 

 pectoral girdle are the scapula and coracoid, both contributing 

 to the glenoid cavity into which the head of the humerus fits. 

 The anterior borders of these bones are characteristically 

 indented. The membrane-bones consist of a pair of clavicles, 

 and a median Y-shaped interclavicle. The forelimb is typical, 

 and similar to that of Triton except that it has five fingers, 

 each ending in claws. 



Pelvic Girdle and Lirnb. — The acetabulum is bordered 

 by ilium, ischium, and pubis. The ilium points backwards 

 towards its articulation with the sacral vertebrae. Both the 

 ischium and the pubis meet their fellow-bones of the opposite 

 side in the middle line, forming symphyses. On each side, the 

 pubis and ischium are separated by the ischio-pubic foramen. 



