A solid silica structure that runs along each side of the raphe in genera such as Frustulia.
Round, F.E., Crawford, R.M. and Mann, D.G. (1990). The Diatoms. Biology and Morphology of the Genera.
Cambridge University Press, Cambridge, 747 pp.
Image Credit: Carrie Graef
In Frustulia, the raphe lies between the two branches of the longitudinal rib.
Image Credit: Sarah Spaulding
Species in Playaensis possess a longitudinal rib on either side of the raphe.