Placoid scales (dermal denticles) are tough scales that cover the skin of elasmobranchs (sharks and rays). Even though placoid scales are similar to the scales of bony fish, they are really modified teeth and are covered with a hard enamel. Placoid scales are packed tightly together and grow with their tips facing backwards. This gives the fish's skin a rough feel. The function of these scales is for protection against predators, although in some sharks, they may also have a hydrodynamic function.
Like our teeth, placoid scales have an inner core of pulp (made up of connective tissues, blood vessels, and nerves), which is covered by a layer of dentine (hard calcareous material). This dentine is covered by enamel-like vitrodentine.
Scales in bony fish grow as the fish gets larger, but placoid scales stop growing after they reach a certain size, and then more scales are added as the fish grows.