A very delicate silica membrane covering a pore and itself perforated by smaller round or elongate pores. Found in many raphid diatoms such as the genus Cocconeis. Plural form is hymenes.
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.