Source : Webster's Revised Unabridged Dictionary (1913)

Hypostome \Hy"po*stome\, Hypostoma \Hy*pos"to*ma\, n. [NL.
   hypostoma, fr. Gr. "ypo` beneath + ? mouth.] (Zo["o]l.)
   The lower lip of trilobites, crustaceans, etc.