Costa (primary, secondary, tertiary)

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.

Costate Glossary
Image Credit: Ian Bishop
Scanning electron micrograph of a valve of Diatoma, showing the interior of the valve. The white arrow shows a primary costa. The black arrow shows a tertiary costa.