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A taxonomical study of 15 Rhoicosphenia species based on light and scanning electron microscope was performed. The genus Rhoicosphenia is characterized by frustule heterovalvy, with a fully developed raphe on the concave valve (referred also as R valve) and reduced raphe on the convex valve (referred as D-valve). One of the most frequently reported species, R. abbreviata, is characterized by narrow, linear valves with low stria density. In contrast, Rhoicosphenia macedonica has broadly clavate valves with a high stria density. According to valve shape R. macedonica is similar to the widely distributed marine species R. marina, but the latter has broader valves (8.5-11 μm wide) and a lower stria density. Both, R. tenuis and R. baicalensis have similar valve sizes and shapes, but the latter is characterized by a lower stria density on the D-valve. Rhoicosphenia baltica is a brackish water species with protracted and subcapitate base poles, narrower valves and lower stria density compared with R. marina. The main distinctive features of R. flexa are the absence of a raphe branch on the head pole of the convex D-valve, and coarsely punctate striae. R. genuflexa has isopolar, linear valves. Four other species have different combinations of characters (valve size, shape and stria density) compared to above mentioned species, and their formal descriptions are provided. Rhoicosphenia lacustris sp. nov. is characterized by larger and broadly clavate valves, densely spaced striae and elongated areolae. Rhoicosphenia affinis sp. nov. has broadly lanceolate valves with attenuated and slightly protracted head pole with a higher stria density compared to R. abbreviata. Rhoicosphenia adriatica sp. nov. is a brackish water species having narrow, linear valves, with a high stria density. Finally, R. omblaensis sp. nov. has a characteristic central area, which appears as a narrow hyaline area around the central pores.

A taxonomical study of Rhoicosphenia Grunow (Bacillariophyceae) with a key for identification of selected taxa.