OF THE PAIRED FINS OF ELASMOBRANCHS. 727 



During the first stage of its development the skeleton of both 

 fins may thus be described as consisting of a longitudinal bar 

 running along the base of tJie fin, and giving off at right angles 

 series of rays which pass into the fin. The longitudinal bar 

 may be called the basipterygium ; and it is continuous in front 

 with the pectoral or pelvic girdle, as the case may be. 



The further development of the primitive skeleton is different 

 in the case of the two fins. 



The Pelvic Fin. The changes in the pelvic fin are compara- 

 tively slight. Plate 33, fig. 2, is a representation of the fin and 

 its skeleton in a female of Scy Ilium stellarc 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 prolongation of the basipterygium, while the 

 anterior ray is united rather with the pelvic girdle than with the 

 basipterygium. All the cartilaginous fin-rays except the first 

 are completely continuous with the basipterygium, their structure 

 in section being hardly different from that shewn in Plate 33, fig. i. 



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 off 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 

 33, fig. 3, shewing the fin of a young male of Scy Ilium 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 off from it and from the pelvic girdle, the 

 posterior end of the basipterygial 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. 



