The Collaborative International Dictionary of English v.0.48:

Placoganoidei \Plac`o*ga*noi"de*i\, n. pl. [NL., fr. Gr. pla`x,
   plako`s, a tablet + NL. ganoidei. See Ganoidei.] (Zool.)
   A division of ganoid fishes including those that have large
   external bony plates and a cartilaginous skeleton.
   [1913 Webster]