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 



**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 pector.il fin is formed 

 of three basal cartilages (bs. 1-3) and numerous radials (raet); 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 



