The Collaborative International Dictionary of English v.0.48:

holocephalan \holocephalan\ n.
   A member of the Holocephali, fish with high compressed
   heads and a bodies tapering off into a long tail. See also
   Hoocephali.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

holocephalan
    n 1: fish with high compressed head and a body tapering off into
         a long tail [syn: holocephalan, holocephalian]