One of the small, thin, membranous, bony or horny pieces which form the covering of many fishes and reptiles, and some mammals, belonging to the dermal part of the skeleton, or dermoskeleton. See Cycloid, Ctenoid, and Ganoid. Fish that, with their fins and shining scales, Glide under the green wave. Milton. Hence, any layer or leaf of metal or other material, resembling in size and thinness the scale of a fish; as, a scale of iron, of bone, etc. One of the small scalelike structures covering parts of some invertebrates, as those on the wings of Lepidoptera and on the body of Thysanura; the elytra of certain annelids. See Lepidoptera. A scale insect. (See below.) A small appendage like a rudimentary leaf, resembling the scales of a fish in form, and often in arrangement; as, the scale of a bud, of a pine cone, and the like. The name is also given to the chaff on the stems of ferns. The thin metallic side plate of the handle of a pocketknife. See Illust. of Pocketknife. An incrustation deposit on the inside of a vessel in which water is heated, as a steam boiler. The thin oxide which forms on the surface of iron forgings. It consists esentially of the magnetic oxide, Fe3O4. Also, a similar coating upon other metals. Covering scale (Zoöl.), a hydrophyllium. -- Ganoid scale (Zoöl.) See under Ganoid. -- Scale armor (Mil.), armor made of small metallic scales overlapping, and fastened upon leather or cloth. -- Scale beetle (Zoöl.), the tiger beetle. -- Scale carp (Zoöl.), a carp having normal scales. -- Scale insect (Zoöl.), any one of numerous species of small hemipterous insects belonging to the family Coccidæ, in which the females, when adult, become more or less scalelike in form. They are found upon the leaves and twigs of various trees and shrubs, and often do great damage to fruit trees. See Orange scale,under Orange. -- Scale moss (Bot.), any leafy-stemmed moss of the order Hepaticæ; -- so called from the small imbricated scalelike leaves of most of the species. See Hepatica, 2, and Jungermannia.