STEUCTUEE 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 tlie rivers of Queensland, 

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



FiG. 49. — Skeleton of tlie Perch, n — f.jaws; (i", eye j e/, portion^ 

 of skull ; ff ff', backbone ; /;, pectoral lin ; /, pcivic fiu; l: /, first 

 and second back-flns ; m, anal fin; ii ;/, tail; 7/ and i are tlie 

 paired fins. 



the bichir (Foh/ptervs) of tiie 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 

 rejiresentative of the lung-fishes, while the second and 

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

 shown in the liichir, represented in Tig. 47, are seen to 

 have a long central lolje 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 lol)e, the skeleton of a fin of 

 this typie being sho^vn in Fig. 50. 



Prom this it will bo seen that such a fin consists inter- 

 nally of a long cartilaginous axis, composed of a number 

 of joints (I — y), and that from one or lioth sides of such 

 joints there are given off obliquely other smallt^r jointed 

 rods terminating in the fine rays forming the free edges 



