The valves are elliptical with slightly convex margins and broadly rounded ends. The valve length is 17.0–22.0 μm, and the valve breadth is 8.0–9.0 μm. The axial area is narrow, linear to lanceolate expanding toward the central area. The central area is round, 2.5–3.5 μm wide. The longitudinal canal is narrow, linear to lanceolate, and expanded in the middle of the valve, with one row of areolae throughout the whole length. The raphe is straight and simple, positioned with an expanded depression. The striae are radiate, 16–20 in 10 μm, composed of round areolae, 25–30 in 10 μm.

Observations: —The observed population is very similar to Diploneis fontanella, from which it differs in stria density (16–20 in 10 μm vs. 16–18 in 10 μm) and areola density (25–30 in 10 μm vs. 22–25 in 10 μm). The valve shape is an additional feature when distinguishing this Arkhangai population from the type (elliptical vs. elliptical to linear-elliptic). In Khentii province Metzeltin et al. (2009) identified a population of D. fontium Reichardt & Lange-Bertalot (2004: pl. 4, fig. 1–6, pl. 5, fig. 1–7), which morphologically is similar to D. aff. fontinella. The valve measurements of the Arkhangai population more closely resemble D. fontanella than D. fontium, therefore we have identified it as D. aff. fontinella. In order to test the conspecificity between these two species, SEM analyses are necessary and should be the subject of further studies.

Ecology and Distribution: —M166A; M167A: not found in Lake Hövsgöl nor previously reported from Mongolia.

Collections are from a small stream in Arkhangai province.