Abstract
The genus Neidiopsis is small comprising eight known taxa distributed in oligotrophic to dystrophic freshwater habitats. During observations of diatoms from alpine intermittent ponds on the Shara Mountain, Republic of North Macedonia, a species of Neidiopsis with high abundance was observed. It is characterized by linear to linear–lanceolate valves with a slightly undulated margin, 25.0–43.5 μm long and 6.0–7.0 μm wide. The raphe is filiform with straight proximal and deflected distal raphe endings to the same side of the valve. Striae are partly biseriate near the axial area and continue as uniseriate circular areolae (ca 40 in 10 μm) towards the valve mantle. Striae on each side of the valve are crossed by a prominent longitudinal line located slightly closer to the valve margin. Ultrastructural features of the type population clearly place this taxon into the genus Neidiopsis and aligned to N. levanderi sensu stricto. A similar documented ecotype population (N. levanderi “calcicola”) sensu Lange-Bertalot (2001) is combined with our population and considered a new species N. borealis sp. nov.