Definition of placoid in US English:



  • (of fish scales) toothlike, being made of dentin with a pointed backward projection of enamel, as in sharks and rays.

    Compare with ctenoid and ganoid
    • ‘Like other sharks, catsharks are covered with placoid scales.’
    • ‘It is an axiom of this theory that hard-tissue units (odontodes in the sense of Orvig 1977) comparable in structure to placoid scales can be inferred to have developed in a directly comparable manner.’
    • ‘Similar attempts had been underway in the dermal skeleton, stemming from Williamson's use of placoid scales from extant sharks as conceptual units in comparative analysis.’
    • ‘The basic odontode unit is identical to the placoid scale of chondrichthyans [SA04] [S + 98].’
    • ‘The earliest attempts to unravel the nature of development and patterning of the vertebrate dermoskeleton and visceral skeleton place central importance upon the nature of the placoid scales in living sharks.’


Mid 19th century: from Greek plax, plak- ‘flat plate’ + -oid.