A silica sheet extending internally from the walls of a copula, or girdle band, occluding a portion of its length. By contrast, a septum is part of a copula, while a pseudoseptum is part of a valve. The septum is found in a number of genera such as Tabellaria. Plural form is septa.
Koppen, J.D. (1975). A morphological and taxonomic consideration of Tabellaria (Bacillariophyceae) from the northcentral United States. Journal of Phycology 11: 236-244.
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.