Placoid scales are found in the cartilaginous fishes . They are also called dermal denticles . Placoid scales are structurally homologous with vertebrate teeth , having a central pulp cavity supplied with blood vessels , surrounded by a conical layer of dentine , all of which sits on top of a rectangular basal plate that rests on the dermis . The outermost layer is composed of vitrodentine , a largely inorganic enamel -like substance. Placoid scales cannot grow in size, but rather more scales are added as the fish increases in size.