xiv. 5 GIRDLES AND LIMBS 375 



The vertebrae can often be divided into four sets. The cervical 

 vertebrae, which are very variable in number, have short ribs, not 

 reaching the sternum. Sternal ribs occur in the thoraco-lumbar seg- 

 ments (Fig. 212). The ribs of the two sacral vertebrae are short and 

 broad and articulate with the ilia. The numerous caudal vertebrae 

 show reduction of all parts, especially towards the tip of the tail. The 

 chevron bones are ossicles attached to the caudal centra and represent- 

 ing the reduced intercentra. 



The girdles and limbs (Figs. 183-6 and 212) show the same general 

 structural and functional features as those of amphibia. The limbs 

 form the main locomotor system, the metachronal contraction of the 

 myotomes playing a lesser part than in urodele amphibians. The 

 humerus and femur are normally held in such a position that their 

 outer ends lie higher than the inner, that is to say, in a position of 

 abduction. The radius and ulna and tibia and fibula proceed down- 

 wards towards the ground (at right angles to the proximal bones) and 

 the hand and foot are turned outwards at right angles, to rest on the 

 ground. The main muscles thus draw the humerus and femur back- 

 wards and forwards as well as downwards, and the ventral regions of 

 the girdles are large and flattened to receive these muscles; in mam- 

 mals, with a different system of progression, the more dorsal parts 

 of the girdles have become developed. 



The pectoral girdle (Figs. 183, 212) consists of a dorsal scapula and 

 a large ventral coracoid, which may be fenestrated. Distinct pro- and 

 post-coracoid elements are probably only found in the extinct mam- 

 mal-like reptiles. The dermal components are represented by the 

 paired clavicles and median interclavicle. A cleithrum is found in a 

 few very primitive forms. 



In the pelvic girdle (Fig. 184) the usual dorsal ilium, anterior pubis, 

 and posterior ischium are found, the last two meeting their fellows in 

 midline symphysis. 



The characteristic modifications of the reptilian skull are discussed 

 on p. 391. The general plan is similar to that of primitive amphibians, 

 but in all except the most primitive reptiles there is a development of 

 holes (fossae) in the temporal region to provide space for the bulging 

 temporal muscles. The skull roof is in some respects more primitive 

 than that in modern amphibians (Figs. 213 and 214). It is made up of a 

 large series of dermal bones, including the nasals, prefrontals, frontals, 

 supra-orbitals, and parietals. The side of the skull is usually less com- 

 plete, composed of the tooth-bearing premaxillaand maxilla, lacrymal, 

 jugal, post-orbital, squamosal, supratemporal and quadrate. The 



