Mentum

from WordNet (r) 3.0 (2006)

mentum
    n 1: a projection like a chin formed by the sepals and base of
         the column in some orchids
    2: the protruding part of the lower jaw [syn: {chin}, {mentum}]
    3: a projection below the mouth of certain mollusks that
       resembles a chin

from The Collaborative International Dictionary of English v.0.48

mentum \men"tum\, n. [L., chin.] (Zool.)
   The front median plate of the labium in insects. See
   {Labium}.
   [1913 Webster]