fold [paleont] . (a) A major rounded elevation on the surface of a brachiopod valve (generally the brachial valve ), externally convex in transverse profile and radial from the umbo, and usually median in position. It is typically associated with the sulcus. Cf: carina. (b) A spirally wound ridge on the interior of the wall of a gastropod shell; e.g. columellar fold and parietal fold.