724 DEVELOPMENT OF THE SKELETON 



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 (inp]. They have 

 been named by Gegenbaur the propterygium, the mesopterygium, 

 and the metapterygium ; and these names are now generally 

 adopted. 



The metapterygium is by far the most important of the three, 

 and in Scy Ilium caniada supports 12 or 13 rays 1 . It forms a 

 large part of the posterior boundary of the fin, and bears rays 

 only on its anterior border. 



The mesopterygium supports 2 or 3 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 attach- 

 ment 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. 



On 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, con- 

 tinues 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 Scy Ilium 

 canicnla and in the male of Scy Ilium catulus (Gegenbaur), 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 rays. It is probably equivalent to 

 two of the posterior rays. 



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

 only 2 rays. 



