SHOULDER-GIRDLE OF VERTEBRATA. 



475 



Fig. 261. Eight half of the shoulder, 

 girdle, and thoracic fin of Gadiis. c Clavicle. 

 a b Supra-clavicles. d Accessory piece, 

 e Coracoid. / Scapula, g Basalia of the fin. 

 h Eays of the secondary skeleton of the fin. 



iii Selachians. In the rest of the Ganoidei and Teleostei a part alone 

 ordinarily remains cartila- 

 ginous, and the rest is ossi- 

 fied, but the whole piece 

 appears to decrease in size. 

 As a rale two bones (/ e) are 

 developed from it in the 

 Teleostei, and with these, parts 

 of the skeleton of the fin 

 may be closely connected. 

 The clavicle, however, which 

 is small in the Sturiones, has 

 increased in size (Fig. 261, c). 

 It is connected along the 

 ventral median line with that 

 of the opposite side, and by 

 the supra-clavicles (a b) with 

 the skull. The primary shoulder-girdle, in fact, undergoes degenera- 

 tion, and forms a mere appendage to the clavicle, which becomes the 

 chief support of the anterior extremity. 



A 

 § 359. 



The clavicle, developed on the carti- 

 laginous shoulder-girdle of Fishes, is 

 reduced in the higher Vertebrata. The 

 primary apparatus, however, becomes of 

 greater importance owing to its con- 

 nection with the sternum, and the greater 

 power of movement possessed by its 

 uppermost (dorsal) portion, which is no 

 longer firmly connected with the axial 

 skeleton. That region of the girdle at 

 which the free limb is connected with it, is 

 distinguished by the formation of a cavity 

 which receives the articular head of the 

 humerus, and divides the primary shoulder- 

 girdle into two parts. 



The dorsal portion forms the scapula ; 

 the ventral is divided into a hinder piece, 

 the coracoid, and an anterior piece, which 

 is ossified from the scapula, when it is 

 ossified — the precoracoid. 



Among the Amphibia, the shoulder- 

 girdle of the Urodela forms a skeletal 

 piece on either side ; it is largely carti- 

 laginous, and is only ossified in the 



region of the glenoid cavity. The widened dorsal end of the 

 scapula, the supra-scapula, is almost always cartilaginous, or has an 



Fig. 262. Shoulder-girdle; A 

 of a Frog, B of a Chelonian. 

 C of a Saurian, s Scapula. 

 s' Supra-scapula. co Precora- 

 coid. co' Coracoid. d Clavicle, 

 c Episternum. st Sternum. The 

 cartilaginous portions are 

 dotted. 



