Source : Webster's Revised Unabridged Dictionary (1913)

Zygosphene \Zyg"o*sphene\, n. [Gr. ? a yoke + ? a wedge.]
   (Anat.)
   A median process on the front part of the neural arch of the
   vertebr[ae] of most snakes and some lizards, which fits into
   a fossa, called the zygantrum, on the back part of the arch
   in front.