The Collaborative International Dictionary of English v.0.48:

Hypostome \Hy"po*stome\, Hypostoma \Hy*pos"to*ma\, n. [NL.
   hypostoma, fr. Gr. "ypo` beneath + ? mouth.] (Zool.)
   The lower lip of trilobites, crustaceans, etc.
   [1913 Webster]