144 



THE OLDEST FISHES AND THEIR FINS. 



lung-fish of Queensland, and the mud-fishes of the rivers 



of Africa and South America, all of which can breathe 



either by gills or by lungs. Another group is formed by 



the so-called ganoid fishes (Fig. 48), 



many of which have the bony armour 



already mentioned; while the great 



majority of the fishes of the present 



day, although nearly related to these 



ancient ganoids, have been generally 



separated as a distinct group, under 



the title of bony fishes. That name 



they take from the circumstance that 



their skeletons are fully ossified, and 



do not partake of the cartilaginous 



nature of those of a shark or a 



ganoid. 



Now if we look at the paired fins 

 (or those which correspond with our 

 own limbs) of any ordinary bony fish, 

 such as the perch (Fig. 49), we shall 

 see that they are formed of a number 

 of bony rays, starting from a single 

 point of origin, and thence spreading 

 out in a fan-like manner. We shall 

 also not fail to observe that the tail of 

 such a fish has a very similar kind of 

 structure, likewise consisting of bony 

 rays, symmetrically arranged, and 

 starting from a curved line where the 

 scales suddenly stop. We may also see 

 from the figure of the skeleton of 

 such a fish that the backbone likewise 

 stops suddenly where the tail begins, 

 and that the rays of the latter start 

 from the expanded end of the back- 

 bone itself. 



The same general type of structure 

 obtains in the paired fins of some of 

 the later ganoids, such as the living 

 sturgeons, as well as certain extinct forms, and it is also 



