frustule

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The hard, siliceous bivalve shell of a diatom.

Century Dictionary and Cyclopedia

1. n. A small fragment.
2. n. The silicious shell of a diatom; a testule. It consists of two valves, one somewhat larger than the other, and closing over it like the lid of a box. The back of each valve is called the side of the frustule; the surface marked by the line of juncture, the front. See cut under Diatomaceæ

Wiktionary

1. n. botany The siliceous shell of a diatom.

GNU Webster's 1913

1. n. (Bot.) The siliceous shell of a diatom. It is composed of two valves, one overlapping the other, like a pill box and its cover.