Light and scanning electron microscopic observations on Epithemia argentina Brun (Bacillariophyceae: Rhopalodiales) and consideration of its systematic position

We compared type material of Epithemia argentina described by Brun, as well as other collections with type material of two purported synonymous taxa, E. debyi and E. multicostata, described by Pantocsek, based on light and scanning electron microscopy. All three taxa were transferred to the genus Rhopalodia by Müller. R. argentina has robust costate fibulae, a large tubular canal raphe, striae in single to double rows of areolae, and volate occlusions with a single point of attachment and positioned internally. R. argentina is distinguished from R. debyi and R. multicostata by costate fibula density and shape of the valves. The latter two species are considered heterotypic synonyms of each other. The distributions of these species over time and space are discussed.