A pointed, hyaline area at the end of a valve, formed by the joining of the longitudinal ribs that lie along each side of the raphe, surrounding the distal end of the raphe. Found in the genus Frustulia. From French for pencil holder.
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.