The Collaborative International Dictionary of English v.0.48:

Orthognathous \Or*thog"na*thous\, a. [Ortho- + Gr. gna`qos the
   jaw.] (Anat.)
   Having the front of the head, or the skull, nearly
   perpendicular, not retreating backwards above the jaws; --
   opposed to prognathous. See Gnathic index, under
   Gnathic.
   [1913 Webster]