In genera such as Diatoma, Meridion, and Tetracyclus, costae can be qualified by the extent to which they traverse the valve face. Primary costae fully traverse the valve. Secondary costae start at the valve margin and end at the sternum. Tertiary costae start at the margin and end before reaching the sternum.
See also 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