60 MR. F. M. BALFOUR ON THE SKELETON 



only occupies a small zone at the base of the fin, the 

 remainder being formed of a fringe supported by radiately 

 arranged horny fibres.^ 



The true skeleton consists of three basal pieces articulating 

 with the pectoral girdle ; on the outer side of which there is 

 a series of more or less segmented cartilaginous fin-rays. 

 Of the basal cartilages one (pp) is anterior, a second (mep) 

 is placed in the middle, and a third is posterior (mp). They 

 have been named by Gegenbaur the proplerygium, the meso- 

 pterygium, and the metapterygium ; and these names are now 

 generally adopted. 



The metapterygium is by far the most important of the 

 three, and in Scyllium canicula supports twelve or thir- 

 teen rays.^ It forms a large part of the posterior boundary 

 of the fin, and bears rays only on its anterior border. 



The mesopterygium supports two or three rays, in the 

 basal parts of which the segmentation into distinct rays is 

 imperfect; and the propterygium supports only a single ray. 



The pelvic fins are horizontally placed, like the pectoral 

 fins, but differ from the latter in nearly meeting each other 

 along the median ventral line of the body. They also differ 

 from the pectoral fins in having a relatively much broader 

 base of attachment to the sides of the body. Their carti- 

 laginous skeleton (woodcut, fig. 2) consists of a basal bar, 

 placed parallel to the base of the fin^ and articulated in front 

 with the pelvic girdle. 



On its outer border it articulates with a series of carti- 

 laginous fin-rays. I shall call the basal bar the basipterygium. 

 The rays which it bears are most of them less segmented 

 than those of the pectoral fin, being only divided into two; 

 and the posterior ray, which is placed in the free posterior 

 border of the tin^ continues the axis of the basipterygium. 

 In the male it is modified in connection with the so-called 

 clasper. 



The anterior fin- ray of the pelvic fin, which is broader than 

 the other rays, articulates directly with the pelvic girdle, 

 instead of with the basipterygium. This ray, in the female 

 of Scyllium canicula and in the male of Scyllium catulus 

 (Gegenbaur), is peculiar in the fact that its distal segment 

 is longitudinally divided into two or more pieces, instead of 



• The horny fibres are mesoblastic products; they are formed, in the first 

 instance, as extremely delicate fibrils on the inner side of the membrane 

 separating the epiblast from the mesoblast. 



' In one example where the metapterygium had 13 rays the mesoptery- 

 gium liad only 2 rays. 



