612 



PAIRED FINS OF ELASMOBRANCHII. 



extends into the fin, and which becomes very early segmented 

 into a series of parallel rays at right angles to the longitudinal 

 bar. 



In other words, the primitive skeleton of both the fins 

 consists of a longitudinal bar running along the base of the fin, 



P-9- 



FIG. 344. PECTORAL FIN OF A YOUNG EMBRYO OF SCYLLIUM IN LONGI- 

 TUDINAL AND HORIZONTAL SECTION. 



The skeleton of the fin was still in the condition of embryonic cartilage. 

 b.p. basipterygium (eventual metapterygium) ; fr. fin rays; p.g. pectoral girdle in 

 transverse section ; /. foramen in pectoral girdle ; pc. wall of peritoneal cavity. 



and giving off at right angles series of rays which pass into the 

 fin. The longitudinal bar, which may be called the basiptery- 

 gium, is moreover continuous in front with the pectoral or 

 pelvic girdle as the case may be. 



The primitive skeleton of the pectoral fin is shewn in 

 longitudinal section in fig. 344, and that of the pelvic fin at a 

 slightly later stage in fig. 345. 



A transverse section shewing the basipterygium (inpt) of the 

 pectoral fin, and the plate passing from it into the fin, is shewn 

 in fig. 346. 



Before proceeding to describe the later history of the two 

 fins it may be well to point out that their embryonic structure 

 completely supports the view which has been arrived at from 

 the consideration of the soft parts of the fin. 



My observations shew that the embryonic skeleton of the 

 paired fin consists of a series of parallel rays similar to those 

 of the unpaired fins. These rays support the soft part of the fin 

 which has the form of a longitudinal ridge, and are continuous 

 at their base with a longitudinal bar, which may very probably 



