x PECTORAL FIN 443 



arch. On its outer surface it presents three articular 

 facets on either side for the corresponding pectoral fin ; the 

 presence v of these allows of the division of each side of the 

 arch into a narrow, pointed, dorsal portion corresponding 

 to the scapular region of the frog (pet, g), and a broader 



bs 

 bsz. 



FIG. 115. Ventral view of pectoral arch of Scy Ilium canicula with right pectoral fin. 

 The pectoral arch is divisible into dorsal or scapular (pet. g), and ventral or cora- 

 coid (pct.g 1 ) portions, continuous at the articular facets (art.f} for the fin. 

 The pectoral fin is formed of three basal cartilages (bs. i 3) and numerous 

 radials (rod) ; its free edge is supported by dermal rays (d.f. r). (After Marshall 

 and Hurst, slightly modified.) 



ventral portion, answering to the coracoid (pct.g) united 

 in the middle line with its fellow of the opposite side (com- 

 pare p. 46) : there is no sternum. The pectoral fin is 

 formed of pterygiophores (rad\ fused proximally to form 

 basals which are three in number (bs. i 3), the third, 

 like the main basal of the pelvic fin, being the largest and 

 supporting the greater number of the cartilaginous rays, 

 which give rise distally to a series of polygonal plates. 



