The Collaborative International Dictionary of English v.0.48:

Perithecium \Per`i*the"ci*um\, n. [NL., fr. Gr. peri` around + ?
   box.] (Bot.)
   An organ in certain fungi and lichens, surrounding and
   enveloping the masses of fructification. --Henslow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

perithecium
    n 1: flask-shaped ascocarp