OF THE PAIRED FINS OF ELASMOBRANCHS. 63 



continuous in front with the pectoral or pelvic girdle, as the 

 case may be. 



The further development of the primitive skeleton is 

 diiferent in the case of the two fins. 



The Pelvic Fin. — The changes in the pelvic fin are com- 

 paratively slight. Plate IV, fig. 2 is a representation of the 

 fin and its skeleton in a female of Scy Ilium stellar e shortly 

 after the primitive tissue is converted into cartilage, but 

 while it is still so soft as to require the very greatest care in 

 dissection. The fin itself forms a simple projection of the 

 side of the body. The skeleton consists of a basipterygium 

 (bp), continuous in front with the pelvic girdle. To the 

 outer side of the basipterygium a series of cartilaginous fin- 

 rays are attached — the posterior ray forming a direct pro- 

 longation of the basipterygium, while the anterior ray is 

 united rather with the pelvic girdle than with the basi- 

 pterygium. All the cartilaginous fin-rays except the first are 

 completely continuous with the basipterygium, their struc- 

 ture in section being hardly different from that shown in 

 Plate IV, fig. 1. 



The external form of the fin does not change very greatly 

 in the course of the further development ; but the hinder 

 part of the attached border is, to some extent, separated ofl 

 from the wall of the body, and becomes the posterior border 

 of the adult fin. With the exception of a certain amount of 

 segmentation in the rays, the character of the skeleton 

 remains almost as in the embryo. The changes which take 

 place are illustrated by Plate IV, fig. 3, showing the fin of 

 a young male of Scyllium stellare. The basipterygium has 

 become somewhat thicker, but is still continuous in front 

 with the pelvic girdle, and otherwise retains its earlier 

 characters. The cartilaginous fin-rays have now become 

 segmented oft' from it and from the pelvic girdle, the poste- 

 rior end of the basiterygial bar being segmented off as the 

 terminal ray. 



The anterior ray is directly articulated with the pelvic 

 girdle, and the remaining rays continue articulated with the 

 basipterygium. Some of the latter are partially segmented. 



As may be gathered by comparing the figure of the fin at 

 the stage just described with that of the adult fin (woodcut, 

 fig. 2), the remaining changes are very slight. The most 

 important is the segmentation of the basipterygial bar from 

 the pelvic girdle. 



The pelvic fin thus retains in all essential points its 

 primitive structure. 



The Pectoral Fin. — The earliest stage of the pectoral fiu 



