Fine structure of two Hygropetra species,Hygropetra gelasina sp. nov. and Hygropetra balfouriana (Bacillariophyceae), and the taxonomic position of the genus with special reference to Frankophila

A small diatom, Hygropetra gelasina sp. nov., obtained from wet moss is described. This diatom is similar to Hygropetra balfouriana (Grunow ex Cleve) Krammer & Lange‐Bertalot, which was found in the same moss sample. Fine structural observations revealed that H. gelasina has a reduced raphe slit and depressions along the margin of the axial area, at the proximal ends of the striae. Both species are characterized by multiple rows of areolae in each stria and a hexagonal pattern of sub‐pores similar to that in Pinnularia, but differing in the position of the areola occlusions or hymenes, which are internal in Hygropetra. Comparison with Frankophila, which shares the characteristics of reduced raphe slits and areola structure with Hygropetra, provides a reference for future taxonomic study of these related genera.