STRUCTURE OF FINS. 



145 



universally present in all the living sharks and rays. If, 

 however, we were to go to the Natural History Museum 

 and examine the lung-fish of the rivers of Queensland, 

 or the gar-pike (Lepidosteus) of those of North America, or 



FIG. 49. Skeleton of the Percli. a r, jaws; d, eye ; apportions 

 of skull ; g ff', backbone ; h, pectoral fin ; /', pelvic fin ; Jc I, first 

 and second back-fins ; m, anal fin ; n n\ tail ; li and i are the 

 paired fins. 



the bichir (Polypterus) of the upper Nile and the rivers 

 of western equatorial Africa, we should find a totally 

 different structure obtaining in the paired fins. In all these 

 three fishes (of which, be it carefully noted, the first is a 

 representative of the lung-fishes, while the second and 

 third are ganoids) the first pair, or pectoral fins, as is well 

 shown in the bichir, represented in Fig. 47, are seen to 

 have a long central lobe running for some distance up the 

 middle of the fin, and completely covered with scales, 

 while the rays of these fins form a kind of fringe, radiating 

 on all sides from the central lobe, the skeleton of a fin of 

 this type being shown in Fig. 50. 



From this it will be seen that such a fin consists inter- 

 nally of a long cartilaginous axis, composed of a number 

 of joints (1 9), and that from one or both sides of such 

 joints there are given off obliquely other smaller jointed 

 rods terminating in the fine rays forming the free edges 



