FISHES. 



115 



they form merely a pair of paddles, as in the extinct Pter- 

 ichthys. 



The highest type of pectoral limb presented by any of the 

 fishes, is that exhibited by Ceratodus (fig. 489), in which 

 there is a median cartilaginous axis, 

 formed by a succession of joints, which 

 in turn support on each side a lateral 

 series of jointed branches, these finally 

 bearing the fin-rays. It seems certain 

 that the " Crossopterygious " Ganoids 

 of the Devonian and Carboniferous 

 periods possessed a similarly - con- 

 structed pectoral fin. 



The hind-limbs or ventral fins are 

 wanting in many fishes, and they are 

 less developed and less fixed in posi- 

 tion than are the pectorals. In some 

 cases the ventral fins are " abdominal " 

 in position, and are placed more or 

 less towards the hinder part of the 

 body (as in the Sharks, Ganoids, and 

 Mud-fishes). In other cases, they are 

 "thoracic" — that is, they are placed 

 beneath the pectorals ; and in some 

 cases they are situated on the sides of 

 the neck in advance of the pectorals, 

 when they are said to be "jugular." 

 In these cases the pelvic arch is at- 

 tached to the pectoral arch, and is 

 therefore wholly removed from its 

 normal position. 



In addition to the pectoral and ven- 

 tral fins — the homologues of the limbs 

 — which may be wanting, fishes are 



furnished with certain other expansions of the integument, 

 which are " median " in position, and must on no account be 

 confounded with the two " paired " fins. These median fins 

 are variable in number, and in some cases there is but a 

 single fringe running round the posterior extremity of the 



Fig. 4S9. — Skeleton of the 

 pectoral fin of Ceratodus, show- 

 ing the median axis and diver- 

 gent brandies on each side, c, 

 Cariaal cartilage. (After Gun- 

 ther.) 



