THE TECTOEAL ARCH. 



35 



continuous cartilage, on each side of the bod}^, distinguish- 

 able only into regions and processes, and affording an articular 

 surface to the bones or cartilages of the limb. But ossifica- 

 tion usually sets up in the cartilage, in such a way as to give 

 rise to a dorsal bone, called the scapula^ or shoulder-blade, 

 which meets, in the articular, glenoidal cavity for the hu- 

 merus, with a ventral ossification, termed the coracoid. 



By difl'erences in the mode of ossification of the varioui? 

 parts, and by other changes, that region of the primitivelj? 



ear 



■m.cr 



Fig. 12. — Side-view of the pectoral arch and sternum of a Lizard {Iguana fuderculata). — 

 )&, scapula ; s.sc, supra-scapula ; cr, coracoid ; gl, glenoidal ca\ity ; St, sternum ; a:8t, 

 xiphisternum ; ???..?c, mesoscapnla; p.cr, precoracoid; m.cr, mesocoracoid ; e.<»', epi- 

 coracoid ; cl, clavicle ; i.cl, interclavicle. 



cartilaginous pectoral arch which lies above the glenoidal 

 cavity may be ultimately divided into a scapula and a supt'a- 

 scajnda / Avhile that which lies on the ventral side may pre- 

 sent not only a coracoid^ but a precoracoid and an epicora- 

 coid. 



In the great majority of the Vertehrata above fishes, the 

 coracoids are large, and articulate with the antero-external 

 margins of the primitively cartilaginous sternum^ or breast- 

 bone. But, in most mammals, they do not reach the sternum, 

 and, becoming anchylosed with the scapula, they appear, in 

 adult life, as mere processes of that bone. 



Numerous Vertebrates possess a clavicula, or collar-bone, 

 which is connected with the pre-axial margin of the scapida 

 and coracoid^ but takes no part in the formation of the 

 glenoid cavity, and is usually, if not alwaj's, a membrane 

 bone. In many Vertehrata, the inner ends of the clavicles 



