A small, toothlike scale, characteristic of cartilagenous fish (e.g., sharks and rays). They have a projecting spine and a flattened base that is embedded in the skin. Placoid scales are made of dentine (no, not the gum) and have a pulp cavity. The spine is covered with a layer of enamel. Shark teeth are probably modified placoid scales.

Plac"oid (?), a. [Gr. , , a tablet + -oid.] Zool.

Platelike; having irregular, platelike, bony scales, often bearing spines; pertaining to the placoids.

© Webster 1913.

Plac"oid, n. Zool. (a)

Any fish having placoid scales, as the sharks.

One of the Placoides.

© Webster 1913.