Source : Webster's Revised Unabridged Dictionary (1913)

Placoid \Plac"oid\, a. [Gr. ?, ?, a tablet + -oid.] (Zo["o]l.)
   Platelike; having irregular, platelike, bony scales, often
   bearing spines; pertaining to the placoids.

Placoid \Plac"oid\, n. (Zo["o]l.)
   (a) Any fish having placoid scales, as the sharks.
   (b) One of the Placoides.

Source : WordNet®

placoid
     adj : as the hard flattened scales of e.g. sharks [syn: {platelike}]