A pseudoseptum is a silica plate extending internally from the apical portion of the valve. To contrast, a pseudoseptum is part of a valve while a septum is part of a copula, or girdle band. For example, the pseudoseptum occurs in some species of Gomphonema, Gomphoneis, Stauroneis and Navicula. The plural is pseudosepta.

Gomphonema Pseudoseptum
Image Credit: Sarah Spaulding
Scanning electron micrograph of a Gomphonema showing an internal view of the valve. At the arrow, the pseudoseptum is the portion of silica extending from the apex of the valve mantle towards the interior of the valve.