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A frustule is the siliceous part of a diatom cell wall. In Latin frustule means a little piece.
A frustule consists of two halves each called a theca (plural is thecae). A theca consists of a valve and, in most taxa, its associated cingulum. The larger epitheca overlaps the smaller hypotheca similar to the overlapping of the halves of a pill box or a Petri dish.
See also epitheca, hypotheca, valve, and cingulum.