Source : Webster's Revised Unabridged Dictionary (1913)

Hyporhachis \Hy`po*rha"chis\, n.; pl. {Hyporhachides}. [NL., fr.
   Gr. "ypo` beneath + ? spine.] (Zo["o]l.)
   The stem of an aftershaft or hypoptilum. [Written also
   {hyporachis}.]