Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

perithecium
     n : flask-shaped ascocarp
     [also: {perithecia} (pl)]