THE ELASMOBRANCII FISHES 



77 



radials {ra., fig. 82). In some forms there may also be found (3) a series of 

 distal plate-like radials {Heterodontus) between upper and lower dermal rays. 



A knowledge of the development of such a skeleton is helpful to an under- 

 standing of the adult frame- 

 work. In Scyllium canicula 

 (fig. 81) Balfour (1881) has 

 shown that the pectoral skele- 

 ton arises as a horizontal bar or 

 jilate of cartilage from which 

 radials (ra., fig. 81) extend. 

 These radials by fusion at their 

 outer tips form a rim from 

 which plate-like distal radials 

 ]iass well out into the fin. As 

 growth progresses the original 

 bar of cartilage becomes the 

 main axis of the fin skeleton, 

 the metapterygium {mt.p., fig. 

 81a) to which numerous ra- 

 dials are attached. Anterior to 

 the metapterygium, the so- 

 called median piece or mesop- 

 terygium (nis.p.) arises sec- 

 ondarily; fewer radials pro- 

 ject from it. There is next seg- 

 mented off from the anterior 

 part of the mesopterj-gium a 

 piece, the propterygium (pr.p., 

 fig. 81b), which bears a single 

 plate-like radial. As for the 

 pectoral girdle, it is formed 

 secondarily from the anterior 

 tip of the horizontal bar. 



In the adult shark the sim- 

 plicity of plan characteristic 

 of this embryonic fin is rarely 

 retained (Chlamj/doselachus) , 

 yet the fundamental plan here 

 laid down is the same, even in 

 the most specialized of pec- 

 torals. Propterygium, meso-, 

 and metapterygium are usu- 

 ally present. The propteryg- Fig. 83. The adult pectoral fin, Ehinohatis produc- 

 • ' \. n ^ -ii li tus. (Mildred Bennett, del.) 



mm may be fused with the J^_^ mesopterygium ; mt.p., metapterygium; 



meS0])teryginm. as in the adult ne.p., neopterygium ; pr.p., propterygium. 



