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a. Relating to or being a kind of fish scale that is a toothlike plate with a protruding spine, characteristic of sharks, skates, and rays.
b. Having placoid scales.
1. (Biology) platelike or flattened
2. (Zoology) (of the scales of sharks and other elasmobranchs) toothlike; composed of dentine with an enamel tip and basal pulp cavity
[C19: from Greek plac-, plax flat]
platelike, as the scales or dermal investments of sharks.
[1835–45; < Greek plak- (s. of pláx) something flat, tablet]