Observations on the type population of Epithemia rupestris W. Smith (Rhopalodiaceae, Bacillariophyta) using light and scanning electron microscopy

Light and scanning electron microscopical observations on the type population of E. rupestris W. Smith are presented. Light microscopy shows valves that are asymmetrical to the apical axis, a canal raphe positioned along the dorsal margin, and costate fibulae that extend across most of the valve. In girdle view, ‘capitate costae' are evident. Scanning electron microscopy shows the valve to have a raphe with a distinct keel bordered by wide openings. Striae are very fine (ca 50 in 10 µm) and are shown to lack volae externally and internally. These observations indicate that this species is morphologically more similar to species in the genus Rhopalodia O. Müller than to the genus Epithemia Brébisson ex Kützing. The lack of volae in R. rupestris (W. Smith) Krammer is unique among rhopalodioid diatoms. Variations in the areolar occlusions across the Rhopalodiaceae are discussed.