The Collaborative International Dictionary of English v.0.48:

Siphonium \Si*pho"ni*um\, n.; pl. Siphonia. [NL., from Gr.
   ????, dim. of ????. See Siphon.] (Anat.)
   A bony tube which, in some birds, connects the tympanium with
   the air chambers of the articular piece of the mandible.
   [1913 Webster]