204 THE STORY OF FISH LIFE. 



like or comb-like shape of its teeth. The ancient 

 creature upon whom this name has been im- 

 posed is one of the very ancient and primitive 

 sharks. Its discovery has done much to enlighten 

 us on the vexed question of the evolution of 

 paired limbs. These appendages in this early 

 type are little more than triangular folds of skin 

 strengthened from within by supports in the 

 shape of rods of cartilage (fig. 17 A). How 

 these rods, by fusion and other modifications, 

 probably formed the foundation of the modern 

 fin we have already discussed (chap, vi,, p. 67), 



Of a somewhat more advanced type, and of a 

 somewhat later date— the Carboniferous and lower 

 Permian— is the form known as Pleuracanthus 

 (fig. 17 C). The fins are now much more ad- 

 vanced in type, but like those of Cladoselache, 

 have formed the subject of much speculation. 

 From its general form and the structure of its 

 fins, this fish looks as though it might, as Dr 

 Smith Woodward points out, with very little 

 modification, become either a shark, lung-fish, or 

 one of the fringe-finned fishes. 



No less remarkable are some small shark-like 

 fish, also of the Carboniferous period, known as 

 Acanthodii— the spiny ones (fig. 17, B.). Their 

 claim to special notice is a strong one, inasmuch 

 as the fins are of a type that is quite unique. They 

 appear to have been derived by specialisation of the 

 type seen in Cladoselache, which has resulted in a 

 fusion of certain of the cartilaginous rays to form 

 a single support at the front of the fin, the rest of 

 the fin was formed by skin only stretched between 

 this support and the body. As in Pleuracanthus, 



