The Collaborative International Dictionary of English v.0.48:

Mesognathous \Me*sog"na*thous\, a. [Meso- + Gr. gna`qos jaw.]
   (Anat.)
   Having the jaws slightly projecting; between prognathous and
   orthognathous. See Gnathic index, under Gnathic.
   [1913 Webster]