60 



PICTURESQUE SKETCHES 



such as need detain us here, there is another change 

 of a far more striking character. I mean the intro- 



Fig. 16 Fig. 17 



MEGALODON. 



duction and speedy increase of the great natural class 

 of fishes, whose remains are comparatively rare even 

 in the uppermost silurian rocks, but which become ex- 

 tremely abundant in those beds immediately superja- 

 cerit. The description of these fishes will be the chief 

 subject of the remaining part of the present chapter. 



Fig. 18 Fig. 19 



Ctenoid Scale PERCH. Cycloid Scale EXTINCT SALMON. 



(Osinerdides Lewesienses.) 



All the fishes at present known to exist in the waters 

 which cover our globe may, with comparatively few 

 exceptions, be grouped naturally and properly in two 

 divisions, the one containing those species whose scales 



