PECTORAL GIRDLE REPTILES 



249 



ing the development of their bones. What is known is summarized 

 below. Even among living reptiles there is difficulty in deciding 

 which has the more primitive girdle. 



Rhynchocephalia. — Sphenodon, whose development is known, 

 has each half of the early pectoral girdle as a continuous cartilage, 

 perforated for the supracoracoid nerve (fig. 264). The cartilage 

 extends to the clavicle and transverse arm of the episternum, and 

 lacks gaps or fenestrae. The medial margins of the two sides meet in 

 front, dorsal to the episternum, while the diverging posterior borders 

 enclose the anterior sides of the sternum. 



Ossifications in this cartilage form scapula and coracoid which 

 fuse at the glenoid fossa. Aside from these there are no cartilage 

 bones and not all of the cartilage is utilized in these, the rest of it 



Fig. 264. — A and B, development of pectoral girdle of Sphenodon before and at the 

 appearance of bone (Howes and Swinnerton, 'oi); C, adult (Furbringer, 'oo). c, cora- 

 coid; cl, clavicle; cs, coraco-scapular fenestra; ec, epicoracoid; 6'^, episternum; g, glenoid 

 fossa; sc, scapula; sf, supracoracoid fenestra; 55, suprascapula; st, sternum. 



calcifyinp. The scapula is elongate rectangular, the clavicles are 

 slender curved rods connecting scapula and episternum, the latter 

 being T-shaped, the cross bar of the T being short and joining the 

 posterior sides of the claviculas. 



Comparisons with Anura and Lacertilia make it probable that the part 

 lettered ec, in figure 264 must be the epicoracoid, while that next the clavicle 

 and lateral arm of the episternum is the precoracoid part. These relations can 

 be carried to fossils like PalcEohatteria and Palaosaurus, and from these it would 

 appear that the interpretation of the Stegocephalan girdle (p. 246) is correct, 

 allowances being made for the non-fossilization of the cartilage. The Thalat- 

 tosauria had an oval plate-like coracoid and a narrow scapula. 



Squamata. — A pectoral girdle occurs only in Lacertilia and 

 Pythonomorphs, snakes having lost it completely, while apodal 

 Hzards have it greatly reduced, or in a few genera entirely absent, all 

 stages of reduction occurring in existing forms. In typical lizards 



