34 THE STORY OF FISH LIFE. 



middle ages, and likewise long since out of fashion, 

 or rather out of harmony with the times. The 

 need for such has not passed, but the end it was 

 intended to serve is now no longer attainable, 

 and hence its disappearance like all other useless 

 structures. What led to the disappearance in 

 the fish we cannot say. 



It is interesting to note here that at one time 

 the character of the scale was an important 

 feature in the classification of fishes. Thus, the 

 horny, disc-like scale of the roach was known 

 as a cycloid scale. When scales of this type 

 possessed a number of fine tooth-like processes 

 along the free edge, they were known as 

 " ctenoid," or comb-like scales. When they took 

 the form of thick square plates with an enamel- 

 like surface, they wire described as ganoid; and 

 when they were of the form which we have seen 

 in the sharks and rays, they were called 

 "placoid." It was supposed that these various 

 forms could be regarded, more or less truthfully, 

 as representing at least three distinct types of 

 fishes. Thus the cycloid and ctenoid scales were 

 held to be typical of the higher fishes ; the ganoid 

 of an intermediate type ; and the placoid of the 

 lowest type of fishes. This is now known to be 

 an erroneous view. ^Etheolepis has been shown 

 by Dr Smith Woodward to possess cycloid scales. 

 Moreover these, by a gradual change of form, 

 pass from the cycloid to the characteristic rhombic 

 plate with peg and socket joints characteristic of 

 the "ganoids." 



This study of the scales of fishes provides us 

 with an interesting lesson in evolution. Thus, 



