SKELETON. 



169 



enforced by membrane bones. The chief and largest of these 



is the cleithrum (usually called the clavicle), developed on the 



outer anterior surface of the girdle, the cleithra of 



the two sides frequently uniting below. To this is 



added above a supraclavicle, which may connect 



directly, or by the intervention of a posttemporal 



bone, with the base of the skull. Other membrane 



bones — postclavicle, infraclavicle, etc. — sometimes 



occur. 



In the amphibia and 

 higher groups other portions 

 may be differentiated in the 

 pectoral girdle, and as yet 

 these cannot all be homolo- 

 gized with the conditions 

 found in fishes. In fact, it 

 is probable that no detailed 

 homology exists. The scapu- 

 lar portion of the arch may 

 ossify throughout, or the os- 

 sification may be restricted to 



that portion — the scapula — nearest the glenoid fossa, while 

 the dorsal portion may be a distinct element, partly or entirely 

 cartilaginous, — the suprascapula. The ventral portion of the 

 girdle gives rise typically to two elements, a posterior coracoid 



Fig. 179. Shoulder girdle of carp 

 {^Cyprinus carpio'), after Gegenbaur. 

 CZ, cleithrum ; E, scapulare ; PC, cora- 

 coid (procoracoid) ; F, foramen between 

 coracoid and cleithrum; A, attachment 

 of fin. 



Fig. 180. Pectoral girdles of, A, 

 Archegosaurus and, B, Palaohalleria. u, 

 coracoid ; cl, clavicle ; e, episternum ; 

 s, scapula, after Credner. 



Fig. 181. Shoulder girdle, etc., 

 of Bombinalor igneiis, after Wied- 

 ersheim. i, clavicle; co, coracoid; 

 ec, epicoracoid ; g, glenoid fossa ; 

 pc, procoracoid; s, scapula; ss, 

 suprascapula; st, sternum. 



