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Webster 1913 Edition

Noun.

[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.

frustule ‎(plural frustules)

(botany) The siliceous shell of a diatom.
- Frustule
  A frustule is the hard and porous cell wall or external layer of diatoms. The frustule is composed almost purely of silica ...