Scales are the remnants of the bony armour which enveloped the very earliest fish.
Fish have scales as a protective coating for the skin. In fact, not all fish have them. But we usually think of a fish as a cold-blooded, aquatic animal that swims by means of fins, breathes by means of gills, and is covered with scales. Scales may be of four different kinds-placoid, ganoid, cycloid and ctenoid.
Placoid scales are long, spiny and toothlike, and are made of enamel and dentine. These are found on fishes which have a backbone made of gristle, such as sharks and rays.
Ganoid scales are rather like placoid scales but are mainly bony and covered with a kind of enable called ganoin. These thick scales are found especially in garfish.
Cycloid scales are thin, large, round or oval scales arranged in an overlapping pattern. They are found in carps and similar fishes.