664 MR. F. M. BALFOVR OX THE SKELETON [Juiie 7, 



pterygium, the mesopferijgium, aud the metapterygium ; aud these 

 names are now 2;enerally adopted. 



The metapterygium is by tar the most important of the three, and 

 in Scyl/ium canicula supports 12 or 13 rays \ It forms a large part 

 of the posterior boundary of the fin, aud bears rays only ou its 

 anterior border. 



The mesopterygium supports 2 or 3 rays, in the basal parts of 

 which the segmentation into distinct rays is imperfect ; aud the pro- 

 pterygium 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 cartilaginous 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. 



Ou its outer border it articulates with a series of cartilaginous 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 fin, continues the axis of 

 the basipterygium. In the male it is modified in connexion 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 ScyUium canicula and 

 in the male of Scyllium catulus (Gegeubaur), is peculiar in the fact 

 that its distal segment is longitudinally divided into two or more 

 pieces, instead of being single as is the case with the remaining ravs. 

 It is probably equivalent to two of the posterior rays. 



Development of the paired Fins. — The first rudiments of the limbs 

 appear in Scyllium, as in other fishes, as slight longitudinal ridge- 

 like thickenings of the epiblast, which closely resemble the first 

 rudiments of the unpaired fins. 



These ridges are two in number on each side — an anterior imme- 

 diately behind the last visceral fold, and a posterior on the level of 

 the cloaca. In most Fishes thej are in no way connected; but in 

 some Elasmobrnnch embryos, more especially in that of Torpedo, 

 they are connected together at their first development by a line of 

 columnar-epiblast cells. This connecting line of columnar epiblast, 

 however, is a very transitory structure. The rudimentary fins 

 soon become more prominent, consisting of a projecting ridge 

 both of epiblast and mesoblast, at the outer edge of which is a 

 fold of epiblast only, which soon reaches considerable dimensions. 

 At a later stage the mesoblast penetrates into this fold, and the fin be- 

 comes a simple ridge of mesoblast covered by epiblast. The pectoral 

 fins are at first considerably ahead of the pelvic fins in development. 



The direction of the original epithelial line which connected the 



> In one example where the metapterygium had 13 rays the mesopterygium 

 had only 2 rays. 



