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.

Gvstaurosirella Pinnata
Image Credit: Eduardo Morales
An SEM image showing the ultrastructure of the frustule of Staurosirella pinnata in girdle view.
Tetracyclus  Iwb
Image Credit: Ian Bishop
An SEM image of part of a frustule of Tabelllaria flocculosa. A valve, with its marginal spines, is visible at the bottom of the image. Attached to the valve are many of the copulae, or girdle bands, that make up the girdle.