The side of the valve face that is narrower and less convex than the dorsal side. Occurs in genera that are asymmetrical to the apical axis, such as Cymbella and Encyonema. From Latin for belly.
Barber, H.G. and Hayworth, E.Y. (1981). Guide to the Morphology of the Diatom Frustule. Freshwater Biological Association, 112 pp.