The inward expanded plate from the copulum, or girdle band, is called a septum. Septae are present in a number of genera, including Tabellaria. Note that septae differ from pseudoseptae by their sites of origin: septae are attached to copulae, pseudoseptae are attached to valves.
Koppen, J.D. (1975). A morphological and taxonomic consideration of Tabellaria (Bacillariophyceae) from the northcentral United States. Journal of Phycology 11: 236-244.