In some taxa, an internally thickened edge of the axial area is termed the axial costa. Two axial costae may be present, one on each side of the raphe. Axial costae is a term used to refer to structures in Mastogloia smithii, among other taxa. Paddock and Kemp (1990) note that the axial costa forms a ‘gutter’, which widens slightly at the central nodule, but does not extend to the distal raphe ends.
this term that is used to describe similar structures in Frustulia and other genera. In general, a longitudinal rib is rounded and relatively wider than axial costae.
Paddock, T.B.B. and Kemp, K.D. (1990). An illustrated survey of the morphological features of the diatom genus Mastogloia. Diatom Research 5: 73-103.
Ross, R., Cox, E.J., Karayeva, N.I., Mann, D.G., Paddock, T.B.B., Simonsen, R. and Sims, P.A. (1979). An amended terminology for the siliceous components of the diatom cell. Nova Hedwigia, Beihefte 64: 513-533.