The Collaborative International Dictionary of English v.0.48:

Pyxidium \Pyx*id"i*um\, n.; pl. Pyxidia. [NL., fr. Gr. ?, dim.
   a ? a box. See Pyx.] (Bot.)
   (a) A pod which divides circularly into an upper and lower
       half, of which the former acts as a kind of lid, as in
       the pimpernel and purslane.
   (b) The theca of mosses.
       [1913 Webster]

WordNet (r) 3.0 (2006):

pyxidium
    n 1: fruit of such plants as the plantain; a capsule whose upper
         part falls off when the seeds are released [syn:
         pyxidium, pyxis]