A ridge of silica elevated above the valve surface. Formed by a folding of the valve wall. Keels occur in genera such as Nitzschia, Surirella, and Entomoneis.

The keel has the raphe on its outer edge, as in the illustration.

Rhopalodia 2
Image Credit: Ditmar Metzeltin, Marina Potapova, Pat Kociolek
A scanning electron micrograph of a frustule of Rhopalodia. The keel of each valve is indicated by an arrow. The raphe is visible as a faint line along the edge of each keel.