In most biological nomenclature, a scale (Greek lepid) is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration. Scales are quite common and have evolved multiple times with varying structure and function.
Fish scales are bony and covered with a smooth transparent tegument to improve the flow of water over them. Reptile scales are more like fingernail. Birds also have scales, commonly on their feet, and their feathers are thought to have been derived from modified scales. A few mammals also have scales, such as the pangolin, and these are originally derived from hair.
There are various types of scales according to shape and class. The scales of bony fishes are laid head to tail, reducing drag.
Cosmoid scales
True cosmoid scales can only be found on the extinct Crossopterygians. The inner layer of the scale is made of compact bone. On top of this lies a spongy layer and then a layer of dentinelike material called cosmine . The upper surface is keratin. The coelacanth has modified cosmoid scales, that are thinner than true cosmoid scales...
Ganoid scales
Ganoid scales can be found on gars (family Lepisosteidae) and bichirs and reedfishes (family Polypteridae). Ganoid scales are similar to cosmoid scales, but a layer of ganoin lies over the cosmine layer and under the enamel. They are diamond-shaped, shiny, and hard.
Placoid scales
Placoid scales are found on cartilaginous fish and sharks. These scales, also called denticles , are similar in structure to teeth.
Ctenoid and cycloid scales
Ctenoid which have a toothed outer edge, and are usually found on fishes with spiny fin rays, such as bass and crappie.
Cycloid scales have a smooth outer edge, and are most common on fish with soft fin rays, such as salmon and carp.
As they grow, cycloid and ctenoid scales add concentric layers.
Lepidopteran wing scales
Butterfly and moth species of the order Lepidoptera (Greek "scale-winged") have membranous wings covered in delicate, powdery scales. Each scale consists of a series of tiny stacked platelets of organic material. Because the thickness of the platelets is on the same order as the wavelength of visible light the plates lead to structural coloration and iridescence through the physical phenomenon described as thin-film optics.
Others
Reptile scale types include: cycloid, granular (which appear bumpy), and keeled (which have a center ridge).
