The Collaborative International Dictionary of English v.0.48:

Buttonmold \But"ton*mold`\, n.
   A disk of bone, wood, or other material, which is made into a
   button by covering it with cloth. [Written also
   buttonmould.]
   [1913 Webster]

   Fossil buttonmolds, joints of encrinites. See Encrinite.
      [1913 Webster]