The Collaborative International Dictionary of English v.0.48:

Prognathous \Prog"na*thous\ (pr[o^]g"n[.a]*th[u^]s), a. [Gr.
   pro` before + gna`qos the jaw] (Anat.)
   Having the jaws projecting beyond the upper part of the face;
   -- opposed to orthognathous. See Gnathic index, under
   Gnathic.
   [1913 Webster]

         Their countenances had the true prognathous character.
                                                  --Kane.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prognathous
    adj 1: having a projecting lower jaw [syn: prognathous,
           prognathic, hypognathous] [ant: opisthognathous]