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 a hard, enamel-like substance (vitrodentine), and the scale projects outwards and backward. The bulk of the denticle 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 denticles are added instead.