An unornamented, elongated thickening of the valve. Usually refers to a thickening parallel to the striae. Plural form is costae. From Latin for rib.
In some genera, such as Diatoma, Meriodion, and Tetracyclus, costae are further qualified by the extent to which they transverse the valve face. Primary costae fully transverse the valve, whereas secondary costae end before reaching the other side of the valve, and tertiary costae end before reaching the sternum.
See also axial costa.
Williams, D.M. (1985). Morphology, taxonomy and inter-relationships of the ribbed araphid diatoms from the genera Diatoma and Meridion (Diatomaceae: Bacillariophyta). Bibliotheca Diatomologica 8: 1-228.
Williams, D.M. (1987). Observations on the genus Tetracyclus Ralfs (Bacillariophyta) I. Valve and girdle structure of the extant species. British Phycological Journal 22: 383-399. 10.1080/00071618700650451