Dictionary of Ichthyology  

scale = 1) a small, stiff, typically plate-like body in the skin of fishes, serving to protect, colour, and support the body. May be modified into spines, tubercles, bony plates, an exoskeleton, reduced or even lost. Phylogenetically absent in Amphioxi and Cyclostomata, and lost in various examples of the other groups, e.g. Torpedo, Ictalurus. The teleost scale is dermal in origin and consists of two major parts - the outer bony, hyalodentine or sclerite layer and the deeper fibrous section called the lamellar layer, fibrillary plate or basal plate. See placoid scale, ganoid scale, cycloid scale and ctenoid scale (latter two also called leptoid or elasmoid scales). Scales have a central focus or centre, an anterior or oral filed, a posterior or aboral field (visible on the flank), and dorsal and ventral fields. See also annulus, circulus.

Scale position in skin of a teleost, D = derma, B = epidermis, from Kingsley's "Textbook of Vertebrate Zoology, 1906. Photograph by Brian W. Coad.

Canadian Museum of Nature