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. Siphonia \Si*pho"ni*a\, n. [NL.] (Bot.) A former name for a euphorbiaceous genus ({Hevea}) of South American trees, the principal source of caoutchouc.