1. 
2. 
[syn: chin, mentum]
3.  a projection below the mouth of certain mollusks that resembles a chin; 
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]
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