GANOID FISH-SCALES 



and parrot-fish, are all of comparatively recent 

 origin. They are not found in the rocks older 

 than the Cretaceous system. On the other 

 hand, the sharks and dog-fish of to-day are the 

 most ancient kind of fish known to us, remains 

 of shark-like fishes occurring in Silurian strata. 

 But the sharks have soft cartilaginous skeletons, 

 and have only, as a rule, left teeth and spines 



Fig. 177.— The hard bony scales of a Ganoid Fish, (a) Four 

 scales as fitted together on the surface of the fish's body ; 

 (6) two scales turned over to show the ridge by which they 

 lock into one another. 



and the denticles of the skin (shagreen) in the 

 rocks. 



On the other hand there are certain fishes 

 known which have hard bony scales and often 

 great plates of bone on the head. They are 

 often called " Ganoid fishes " on account of 

 their hard smooth bony scales (see Fig. 177), 

 which differ in substance and shape from the 

 thin, flexible scales of common fish. They 



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