The Collaborative International Dictionary of English v.0.48:

Frustule \Frus"tule\, n. [L. frustulum, dim. fr. frustum a
   piece: cf. F. frustule.] (Bot.)
   The siliceous shell of a diatom. It is composed of two
   valves, one overlapping the other, like a pill box and its
   cover.
   [1913 Webster]