The Collaborative International Dictionary of English v.0.48:

Placoid \Plac"oid\, a. [Gr. pla`x, plako`s, a tablet + -oid.]
   (Zool.)
   Platelike; having irregular, platelike, bony scales, often
   bearing spines; pertaining to the placoids.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Placoid \Plac"oid\, n. (Zool.)
   (a) Any fish having placoid scales, as the sharks.
   (b) One of the Placoides.
       [1913 Webster]

WordNet (r) 3.0 (2006):

placoid
    adj 1: as the hard flattened scales of e.g. sharks [syn:
           placoid, platelike]