148 THE OLDEST FISHES AND THEIR ilNS. 



are, indeed, more like clumsy paddles, capable only of 

 comparatively slow and steady motions ; sucli movements 

 being sufiBcient for fislies protected either by the bony 

 armour of the ganoids, or by the spines with which the 

 early sharks (Fig. 51) were armed. The fan-like fin is, 

 therefore, obviously the most specialized type of structure, 

 and as the ganoids m their advance towards the bony 

 fishes gradually acquired this fan-like fin, and with it, we 

 may presume, increased speed, it was essential that their 

 enemies the sharks should follow suit in order to be able to 

 catch their iprej. This would apjjear to be a sufficient 

 reason for the attainment of the fan-like structure of fin in 

 both these groups of fishes. It is, however, very remark- 

 able that this structure of the fins having Iteen iudeisen- 

 dently developed in the two groujss should have become so 

 alike as it is. On the other hand, the lung-fishes, 

 together with the gar-pike and the bichir, never having 

 had occasion to abandon the mud-loving and sluggish 

 habits of their Palseozoic ancestors, have fortunately 

 pi'eserved for us intact the old fringe-finned type of 

 swimming organs. 



It is not, however, in regard to these paired fins alone 

 that fishes show a modification from a long, jointed, axial 

 structure to one which stops suddenly in an exisanded 

 termination, from which arises a fan-shaped arrangement 

 of rajfs, the same kind of modification, although far less 

 generally, having also taken place among fishes in the 

 structure of their tails. 



Thus, in all the primeval fishes the backbone (as shown 

 in Fig. 51) is continued right to the very end of the tail, 

 where it terminates in a point. On either side of the 

 backbone are fringes of fin-rays, so that (as sho'svn in 

 Fig 48) in scaled fislies the scaly part of the tail is con- 

 tinued nearly to its extremity. This type of tail is there- 

 fore exactly similar in structure to the fringe-finned type 

 of fin, and may be similarly known as the fringe-tailed 

 type. In some fringe-tailed fishes the fringes on either 

 side of the tail (as in Fig. 47) are of nearly equal depth. 

 In other instances, however, the fringe of rays on the 

 lower side is somewhat deeper than that on the upper ; 



