A new monoraphid diatom genus, Planoplatessa gen. nov., is described on the basis of a detailed morphological investigation using light and scanning electron microscopy. We transferred Achnanthes joursacensis Héribaud, a species described from fossil deposits in France, to our new genus. We investigated material from Mongolia from Recent populations. This taxon is known from fossils to Recent across the Holarctic. Achnanthes joursacensis was previously transferred to the genera Planothidium and Platessa, but the morphology of A. joursacensis does not share all morphological features with these two genera. We discuss important morphological features for the delimitation of monoraphid genera based on careful morphological and molecular investigations we presented previously. Planoplatessa gen. nov. as a genus is characterized by having uniseriate striae on both the raphe and the rapheless valves, a cavum in rapheless valves only, and straight distal raphe ends on the valve face.
Keywords: Lake Khövsgöl; Mongolia; Planoplatessa; Platessa; diatoms; morphology; new genus.