OF CREATION. 61 



are jagged on the outer edge like the scales of the 

 perch (fig. 18), and the other those whose scales are 

 smooth and simple at the margin, like the scales of a 

 herring or salmon (fig. 19). To the peculiarities thus 

 alluded to, might be added many others derived from 

 the minute anatomy of the fishes. 



20 Fig. 21 



Ganoid Scale BONY PIKE. Placoid Scale EXTINCT RAY. 



(Lepidosteus.) (Spinacorhinus polyspondylus.} 



The exceptions to this arrangement are comprised 

 within a few natural families, of which the sturgeon, 

 the Siluridse or cat-fish, the bony pike of the North 

 American lakes (fig. 20), and some others, form one 

 group ; and the saw-fish, the rays (fig. 21), and the 

 sharks another. These two groups were naturally 

 looked on as of comparatively small importance, 

 so long as only the existing species of fishes were 

 known, for they contain, with the exception of the 

 sharks and rays, but very few species, and these are 

 neither abundant nor widely spread. 



When, however, it was discovered by M. Agassiz, 

 on looking carefully at the numerous species of fish 

 whose fragments are found fossil in the older rocks, 

 that all these, without a single exception, belonged to 

 one or the other of the two groups alluded to, it be- 

 came necessary to reconsider the subject of the classi- 

 fication of fishes, and learn, if possible, the nature and 



