PECTORAL ARCH. 



89 



extensively developed. In all the higher Vertebrates it is formed 

 on the same plan as in Amphibia, and becomes more or less 

 completely ossified. 



It always consists on each side of a cartilaginous or bony 

 dorsal plate (scapula), which curves round the side of the body, 

 and becomes continuous ventrally with two processes an anterior 



FlG. 71. DlAGKAM OF THE GROUND-TYPE OF PECTORAL ARCH MET WITH IX 

 ALL VERTEBRATA, FROM THE AMPHIBIA UP TO MAMMALIA. 



S, scapula ; Co, coracoid ; 67, precoracoid (clavicle) ; ff, humerus. 



FIG. 72. SEMIDIAGRAMMATIC FIGURE OF THE PECTORAL ARCH AXD STEUNUM 



OF Urodela. 



St, sternum ; a, point where the two coracoids overlap ; 67, precoracoid (clavicle) ; 

 SS, suprascapula, shown extended transversely outwards on the left side ; t, bony 

 scapula ; H, humeras. 



FIG. 73. PECTORAL ARCH OF A CHELOXIAX. (Ventral view.) 

 S, scapula ; Co, coracoid ; Co 1 , epicoracoid ; 67, precoracoid (clavicle) ; B, fibrous 

 band between these two elements ; Fc, fenestra between them ; G, glenoid 

 cavity. 



(precoracoid, or clavicle), and a posterior (coracoid) (Fig. 71, 

 , Cl, Co). It always becomes connected ventrally with the 

 sternal apparatus (compare Figs. 38, 39, 74). 



The two coracoid plates either overlap one another in the mid- 

 ventral line, or else their free edges come into apposition and fuse 

 together (Figs. 74 and 38, Co, Co 1 }. 



