The larger theca of the frustule. The epitheca overlaps the hypotheca. A theca is composed of a valve and its associated cingulum. From Greek, theca means sheath or case (plural is thecae), so epitheca means outer sheath.
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.