A type of scale that comprises the basic unit of the hard skin cover of sharks. It consists of a hard base embedded in the skin and a spiny process (cusp); these are covered by vitrodentine. The scale projects outwards and backwards and it is the cusps that are responsible for the roughness of the shark's skin. The bulk of the scale consists of dentine surrounding a central pulp cavity. Unlike the scales of bony fish, placoid scales stop growing after they reach a certain size and new scales are added as the animal grows.
Subjects: Zoology and Animal Sciences — Earth Sciences and Geography.