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.

Image Credit: S. Spaulding
In this SEM micrograph, septa of Tabellaria are marked with arrows. Each septum is attached to a girdle band, or copula. A valve of Tabellaria (internal view) lies between the two copulae.