Source : Webster's Revised Unabridged Dictionary (1913)

Mentomeckelian \Men`to*meck*e"li*an\, a. [1st mental +
   Meckelian.] (Anat.)
   Of or pertaining to the chin and lower jaw. -- n. The bone or
   cartilage forming the anterior extremity of the lower jaw in
   some adult animals and the young of others.