xii PHYLUM CHORDATA 373 



number of slender horny fibres of dermal origin. In the 

 smaller median fins there may be an elongated rod of carti- 

 lage constituting the skeleton, or cartilage may be entirely 

 absent. In the pectoral fin (Fig. 225) the fin- rays are 

 supported on three basal cartilages articulating with the 

 pectoral arch. The latter is a strong hoop of cartilage in- 

 complete dorsally, situated immediately behind the last of 



d.f.. 



FIG 225. Ventral view of pectoral arch of Scyllium with right pectoral fin. The 

 pectoral arch is divisible into dorsal (pet. g) and ventral (pet. g'} portions, 

 separated by the articular facets (art./) for the fin. The pectoral fin is formed 

 of three basal cartilages (bs. 1-3) and numerous radials (rad} ; its free edge is 

 supported by dermal rays (d.f. r). (Modified from Marshall and Hurst.) 



the branchial arches. It consists of a dorsal, or scapular, 

 (pet. g) and a ventral, or coracoid, portion (pct-g')> the 

 coracoid portions of opposite sides being completely con- 

 tinuous across the middle line, while the scapular are sepa- 

 rated by a wide gap in which .the spinal column lies. 

 Between the two portions are the three articular surfaces 

 for the three basal cartilages. The three basal cartilages of 



