Costa (primary, secondary, tertiary)

An unornamented, elongated thickening of the valve. Usually refers to a thickening parallel to the striae. Plural form is costae. From Latin for rib.

Morphogenetically, a costa parallel to the striae is a secondarily thickened virga.

See also axial costa.


Barber, H.G. and Haworth, E.Y. (1981). Guide to the Morphology of the Diatom Frustule. Freshwater Biological Association, 112 pp.

Cox, E.J. (2011). Morphology, cell wall, cytology, ultrastructure, and morphogenetic studies. Overview and specific observations. In: The Diatom World (J. Seckbach and J.P. Kociolek, Eds,), Cellular origin, life in extreme habitats and astrobiology, Vol. 19, pp. 21–45, Springer, Dordrecht.

Cox, E.J. (2012). Ontogeny, homology, and terminology - wall morphogenesis as an aid to character recognition and character state definition for pennate diatom systematics. Journal of Phycology 48: 1–31. 10.1111/j.1529-8817.2011.01081.x