xii PHYLUM CHORDATA 371 



gap in which the spinal column lies. Between the two 

 portions are the three articular surfaces for the three basal 

 cartilages. The anterior basal cartilage is called the pro- 

 pterygium (bs. i), the middle, meso-pterygtum (bs. 2), and the 

 posterior, meta-pterygium (bs. 3). Of these the first is the 

 smallest, and the last the largest. The pelvic Jin has only 

 a single basal cartilage, articulating with the pelvic arch, 



Jbct.g" 



FIG. 2To. Ventral view of pectoral arch of Scyllium with right pectoral fir. The 

 pectoral arch is divisible into dorsal (pet. g.) and ventral (pct.g.) portions 

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

 of three basal cartilages (bs. 1-3) and numerous cartilaginous fin-rays (rad.) ; 

 its free edge is supported by dermal rays (d.f. r.). (Modified from Marshall 

 and Hurst.) 



with which also one or two of the fin rays articulate directly. 

 The pelvic arch is a nearly straight bar of cartilage which 

 runs transversely across the ventral surface of the body, just 

 in front of the cloacal opening. 



The mouth leads into a mouth cavity passing behind 

 into a very wide pharynx (Fig. 202, //$.), into which 



B B 2 



