lit ■ 



PECTORAL FIN 



427 



arch. On each side of its outer surface it presents three 

 articular facets for the pectoral fin ; the presence 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 {pd. g), and a broader 



rfjr 



KlG 106. — \'enlrai \iew of pectoral arch u{ Scy/Z/ii/zi witil right pccloral fin. 

 The pectoral arch is divisible into dorsal or scapular (pct.^) and ventral or cora- 

 cold {pct.,^') portions separated by the articular facets {art. f) for the fin. 

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

 radials t^yad) ; its free edge is supported by dermal rajs (,/, / r). (Modified from 

 Marshall and Hurst.) 



ventral portion, answering to the coracoid ( /rA /) united 

 in the middle line with its fellow of the opposite side : there 

 is no sternum. The pectoral fin is formed of ptery- 

 giophores (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 ra)s, which give 

 rise distally tu a series of polygonal plates. 



