The Collaborative International Dictionary of English v.0.48:

Ceratophyllum \Ceratophyllum\ n.
   the sole genus constituting the family Ceratophyllaceae;
   the hornworts.

   Syn: genus Ceratophyllum.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Ceratophyllum
    n 1: constituting the family Ceratophyllaceae: hornworts [syn:
         Ceratophyllum, genus Ceratophyllum]