Dictionary of Ichthyology
placoid scale = a type of scale, typically thorn-shaped, found on the skin of Elasmobranchii and claspers of Holocephali. Consists of a spine and a basal plate which is usually rhomboidal. It contains a pulp cavity and is composed of an inner layer of dermal bone, a middle layer of dentine and is covered by enamel-like vitrodentine. Also and less preferably called denticle, dermal denticle or odontoid.