The Collaborative International Dictionary of English v.0.48:

Sphenethmoid \Sphe*neth"moid\, a. [Sphenoid + ethmoid.] (Anat.)
   Of or pertaining to both the sphenoidal and the ethmoidal
   regions of the skull, or the sphenethmoid bone;
   sphenethmoidal.
   [1913 Webster]

   Sphenethmoid bone (Anat.), a bone of the skull which
      surrounds the anterior end of the brain in many amphibia;
      the girdle bone.
      [1913 Webster]