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A new freshwater species, Achnanthidium longissimum sp. nov., is described from Jiuzhai Valley, Sichuan Province (Southwest China), based on light and scanning electron microscopy. This species belongs to the ‘Achnanthidium pyrenaicum complex’, based on having transpically elongated areolae and deflected distal raphe fissures. Achnanthidium longissimum sp. nov. possesses a unique combination of features that includes having the longest valves in the genus (nearly 50 µm long), capitate ends, and raphe vestiges near the apices of the rapheless valve. Furthermore, the mantle has a single row of linear areolae, and a narrow hyaline area at the valve face – mantle junction on both valves. Achnanthidium longissimum is compared with morphologically similar species. The new species is known from epilithic and epiphytic collections from three alkaline lakes in Jiuzhai Valley.

This article clarifies the taxonomy and nomenclature of several monoraphid diatom species found in rivers and lakes of North America. The morphology of these diatoms is studied with light and electron microscopy. A new combination Rossithidium anastasiae (Kaczmarska) Potapova is introduced for a species common in both Europe and North America and often misidentified as Achnanthes linearis (W. Smith) Grunow or R. linearis (W. Smith) Round et Bukhtiyarova. Achnanthes harveyi Reimer, often found in eastern North America, is transferred to Psammothidium Bukhtiyarova & Round on the basis of the convex raphe valve, uniseriate striae, and raphe structure. A new Psammothidium species, P. pennsylvanicum Potapova is described and compared with the most similar taxon, P. semiapertum (Hustedt) Aboal. Diatoms originally described as Achnanthes lutheri Hustedt and A. stewartii Patrick are transferred to the genus Platessa Lange-Bertalot on the basis of biseriate striae, flat valves, and the absence of terminal raphe fi...

I am writing a book chapter on the taxonomy and ecology of araphid and monoraphid freshwater diatoms in North America. Some species names in recently described genera belonging to these groups are invalidly published (Fourtanier & Kociolek 1999). Other necessary transfers have not been made. This note is not an exhaustive revision of taxonomy, but rather an effort to ensure that consistent nomenclature is employed in the book chapter. Thanks to Elisabeth Fourtanier, Paul Hamilton, and Mark Edlund for their constructive comments.