FISHES 



349 



fishes 



earth. Consequently, even if at a given time in the past 

 all the now existing families had been evolved, not so 

 many of them would be found in any particular region 

 as today. 



5. The scales of the bony fishes were classified by Scales of 

 Agassiz as cycloid and ctenoid. Any scale which had a 

 circular or oval or squarish outline, with the exposed 

 edge even and free from teeth or spines, was called 

 cycloid. Whenever the exposed margin showed dis- 

 tinct prominences resembling teeth, the scale was called 

 ctenoid or comblike. Agassiz thought that these dis- 

 tinctions separated great groups of fishes, and to some 

 extent he was correct, but it is now known that in many 

 instances species with ctenoid scales are more nearly 

 allied to others with cycloid, than to particular groups 

 in which they are ctenoid. In various flatfishes, the 

 scales are ctenoid on the upper side, cycloid on the lower 

 side. The fact is that while the cycloid condition is 

 undoubtedly the more primitive, it may be secondarily 

 acquired by the loss of the ctenoid features. The case 



After drawing by Max M. Ellis 



FIG. 132. Cycloid scale of Notropis cornutus, an American freshwater fish; 

 showing apical radii.* 



