Source : Webster's Revised Unabridged Dictionary (1913)

Placoderm \Plac"o*derm\, n. [Gr. ?, ?, tablet + ? skin.]
   (Paleon.)
   One of the Placodermi.

Source : WordNet®

placoderm
     n : fishlike vertebrate with bony plates on head and upper body;
         dominant in seas and rivers during the Devonian;
         considered the earliest vertebrate with jaws