Class of unicellular algae, usually occurring singly, but may be colonial or filamentous. Cell size ranges from 5 to 2000 µm. The cell wall (frustule) is impregnated with silica and consists of two valves, one of which overlaps the other like the lid on a box. The frustule is commonly delicately ornamented and pierced by tiny holes (punctae) which may be covered by porous sieve membranes. There are two orders. The Centrales (centric diatoms) are circular, with radial symmetry, and are predominantly marine. The Pennales (pinnate diatoms) are elliptical, with bilateral symmetry, and dominate freshwater environments. The frustules have formed an important constituent of deep-sea deposits since the Cretaceous. The oldest known diatom is usually taken to be Pyxidicula bollensis from the Jurassic. See diatomaceous earth; diatomite; diatom ooze.