PECTOPvAL 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 ventral ly with two processes an anterior 



FIG. 71. DIAGRAM OF THE GROUND-TYPE OF PECTORAL ARCII MET WITH ix 

 ALL VERTEBRATA, FROM THE AMPHIBIA UP TO MAMMALIA. 



8, scapula ; Co, coracoid ; 67, precoracoid (clavicle) ; H, humerus. 



FIG. 72. SEMIDIAGRAMMATIC FIGURE OF THE PECTORAL ARCH AND STERNUM 



OF Urodcla. 



iSt, sternum ; a, point where the two coracoids overlap ; Cl, precoracoid (clavicle) ; 

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

 scapula ; H, humerus. 



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

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

 band between these two elements ; Ft; feiiestra between them ; G, glcnoid 

 cavity. 



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

 S, Cl, Co}. It always becomes connected veutrally 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 }. 



