154 



SELACH11 



fins, but also grow forwards, passing above the branchial slits 

 (Squatinidae, Division 1). Further, the pectoral fins fuse with the 

 sides of the head (Division 2), and may grow forwards, below the 

 eyes and above the mouth and nostrils, to meet in the middle line 

 at the front end of the rostrum. This remarkable development of 

 the pectoral fins can be followed in the ontogeny of young skates 

 (Fig. 117). By the coalescence of radials an anterior endoskeletal 

 axis is developed in the pectoral fin, similar to the original posterior 

 axis (Fig. 121) (Gegenbaur [153], Howes [219]). To support these 



Fig. 117. 



A, B, and C, three successive stages in the development of Torpedo ocellata, Raf. The external 

 gill-filaments have been removed on the left side. ",,, anterior region of pectoral tin growing 

 forward; b. region of fore-brain; br, fourth branchial arch; cl, cloacal aperture; e, eye; e,g, 

 external gills :'/..;, lower jaw ; «., opening of nasal sac ; p, pectoral lin : pv, pelvic fin ; .>-, ridge 

 along which the pectoral fin will grow; sp, spiracle; n.j. upper jaw; ;/. stalk of yolk-sac. 



huge fins, the anterior vertebrae become fused into a continuous 

 tube (Fig. 118), the pectoral girdle becomes firmly attached to it 

 above, by a specially differentiated suprascapula (Fig. 118), and 

 greatly strengthened and widened at the sides for the reception 

 of the basals. The lateral -line system becomes much modified, 

 spreading over the expanded pectoral fins (Fig. 127). The 

 ampullae, also, may be extended over their surface. A very definite 

 articulation, by means of two condyles, is established between the 

 occipital region of the skull and the rigid vertebral column (Fig. 119). 

 The preorbital process of the skull may become very large, forming 

 a separate jointed piece attached to and supporting the pectoral fin 

 (Fig. 1 20). The mouth becomes transverse, and the two halves of 



