The siliceous parts of the diatom cell wall. Composed of the larger epitheca and the smaller hypotheca. The epitheca overlaps the hypotheca similar to a pill box or Petri dish. From Latin for a little piece.

See also valve and cingulum.

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.
Gvstaurosirella Pinnata
Image Credit: Eduardo Morales
An SEM image showing the ultrastructure of the frustule of Staurosirella pinnata.