hyporhachis

from The Collaborative International Dictionary of English v.0.48

Hyporhachis \Hy`po*rha"chis\, n.; pl. {Hyporhachides}. [NL., fr.
   Gr. "ypo` beneath + ? spine.] (Zool.)
   The stem of an aftershaft or hypoptilum. [Written also
   {hyporachis}.]
   [1913 Webster]