x PECTORAL FIN 443 



arch of cartilage situated just behind the last branchial 

 arch. On its outer surface it presents three 'articular 

 facets on either side for the corresponding 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 (pet. g), 



d.f.rt 



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

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

 coid (pet. g') portions, continuous at the articular facets (art. f) for the fin. The 

 pectoral fin is formed of three basal cartilages (bs. 1-3) and numerous radials 

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

 Hurst, slightly modified. ( X f .) 



and a broader ventral portion, answering to the coracoid 

 (pet. g'), united in the middle line with its fellow of the 

 opposite side (compare 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. 1-3), 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. 



It will be noticed that while the skeleton of the crayfish 

 is a series of articulated tubes with the muscles inside 



