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MARINE PRODUCTS OF COMMERCE 



size and structure among the species, however, that they are variously classified. 

 Two main groups of scales— cycloid and ctenoid— are generally recognized. Cy- 

 cloid scales have smooth, unindented free edges, whereas ctenoid scales have 



{Courtesy U. S. Fish and Wildlife Service) 



Fig. 10-5. The porcupine fish {Chilomyctertis schoeffi) . 



rough edges. This division is useful in classifying fishes, even though occasionally 

 scales intermediate in structure and not clearly cycloid or ctenoid are found. 

 Furthermore, some species, like the triggerfish (Balistes) have platelike scales 

 (Fig. 10-6). Then, there are elongate, round, and deep scales of various sizes. 



{Courtesy U. S. Fish and Wildlife Scri'ice) 

 Fig. 10-6. The triggerfish {Balistes bursa). 



The fresh-water eel is well-covered with small elongate scales placed in patches, 

 the scales in one patch at right angles to those in another. Though embedded 

 in the skin they are visible externally upon close examination. The tarpon. Tarpon 

 atlanticus, on the other hand, has very large, overlapping, external scales, which 



