Valves are elliptical-lanceolate becoming almost circular with size reduction, with convex margins and broadly round ends (Figs 80–91, 97–109, 111). The valve length is 19.5–44.5 μm, and the valve breadth is 12.5–22.0 μm. The axial area is linear to lanceolate, expanding towards the round-elongate to rectangular and slightly asymmetric central area. From inside, a thick linear silica plate covers the whole length of the longitudinal canal (Fig. 93). The central area is round-elongate to rectangular and slightly asymmetric, 1.5–4.0 μm wide. Externally, the longitudinal canal is lanceolate to linear, expanded in the middle of the valve with two to three areolae that coalesce into one areola covered with large and complex cribra toward the valve apices (Figs 90, 106, 107, 111, 112). The external openings of the canal are covered with cribrate occlusions similar to those occluding the striae from which they are separated by a thin hyaline area (Figs 90, 91, 109, 110); cribra are located in a depression slightly lower that the rest of the non-porous silica. Internally, the longitudinal canal is covered with silica plate throughout the whole length, forming a “trench” structure housing the raphe slits (Fig. 93). Externally, the raphe is straight and simple with drop-like proximal ends that are deflected to the same side of the valve and positioned within an expanded depression (Figs 90, 95, 96, 109, 110). Towards the central area the raphe branches are located in a slight “depression” below the surface of the rest of the non-porous silica (Figs 90, 95, 109, 111) and surrounded with a silica ridge (Figs 96, 110). Distally, the raphe branches are bent into a short drop-like terminal fissures (Figs 91, 92, 109, 111, 112). Internally, the raphe is straight with simple proximal and distal ends, inserted in a slightly elevated sternum inside the “trench” formed by the longitudinal canal (Fig. 93). The proximal raphe ends are the only raphe structure that reaches the height of the longitudinal canal (Fig. 93). The striae are uniseriate, radiate, 12–15 in 10 μm, composed of round to rectangular areolae, 10–20 in 10 μm. In some valves there is a slight biseriate striae pattern toward the margins (Figs 95, 102). Externally, the alveolate striae open through one row of areolae covered with cribrate occlusions. The cribrate occlusions increase in size and at the same time become more complex in structure toward the valve margins (Figs 90, 94, 95, 109, 110). Internally, the alveoli open through a single linear opening covered with a fine silica layer (Fig. 93).

Type:— MONGOLIA, Lake Hövsgöl (Hövsgöl National Park), Hatgal. Coordinates: 50°28.052’ N; 100°10.336’ E, Ulothrix /epilithic from 0.1 m depth (accession number: M068 A, M. B. Edlund Collection, Science Museum of Minnesota, collected by Mark B. Edlund, Eugene F. Stoermer, and Nergui Soninkhishig, 11 June 1996) (Slide M068 A, ANSP GC-36353, GCM-24055), holotype, designated here; example specimens on Figs 80, 84, 87–89; (Slide 917068, CAS, isotype, designated here; Slide 919072, CAS, paratype, designated here).

Etymology:— The species name honors Dr. Eugene Stoermer, University of Michigan, who initiated the research collaboration with the National University of Mongolia with a collecting expedition in 1996.

Observations: — Diploneis stoermeri appears to be morphologically variable in shape of the central area (rectangular and slightly asymmetric to round) and in the structure of the longitudinal canal (lanceolate, expanded in the middle of the valve to linear-lanceolate and slightly asymmetrical; compare Figs 80–90 with Figs 97–109). Variability in other distinguishing features, such valve shape, the structure of the striae, and the density of striae and areolae was not observed, but based on the above noted variable structures, we consider the Lake Hövsgöl populations to be a morpholgical complex represented by at least two different phenodemes (Figs 80–96, morphotype 1 and Figs 97–112, morphotype 2). Sympatry among these phenodemes makes their separation even more difficult and opens further questions into their identity and phylogenetic relationship. Such morphological patterns might be correlated with /related to physiological, ecological and/or genetic traits. Therefore, additional analyses including paleolimnology, morphometry and genetic information could provide further support in revealing the patterns and mechanisms for such morphological variations within the D. stoermeri complex, or could simply explain the sympatric coexistence of two genotypically distinct entities (Mann 1999).

Diploneis stoermeri can easily be confused with D. paraparma. These two species share similar morphological features, such as the central area, the axial area, and the structure of the longitudinal canal, but differ in striae structure: uniseriate, rarely biseriate towards the valve margins in D. stoermeri and biseriate becoming uniseriate towards the axial area in D. paraparma (compare Figs 80–91 with Figs 70–76). However, using the striae as a distinguishing feature might be a challenge due to the tendency of the cribrate occlusions to become complex in structure towards the valve margins (see Figs 76, 79, 95), which can leave the impression of a biseriate pattern. Hence the cribrate occlusions cover the alveolate striae, the number of areolae is difficult to observe, and sometimes can lead to misinterpretation. Therefore, when using the striae for identification, the uni- and biseriate pattern should be taken as a main feature in separating D. stoermeri from D. paraparma.

Diploneis parma differs from D. stoermeri in: the valve outline (broadly elliptical vs. elliptical-lanceolate); the central area (round vs. rectangular and slightly asymmetric); and the striae structure (uniseriate becoming biseriate towards the valve margins vs. uniseriate, rarely biseriate towards valve margins). Diploneis stoermeri closely resembles D. krammeri, though the stria density separates them [12–15 in 10 μm vs. 11(12) in 10 μm]. Additionally, the structure of the longitudinal canal further supports the differences between D. krammeri and D. stoermeri (one to two rows of areolae in the middle of the valve narrowing into one at the valve apices vs. two rows of areolae continuing into one that is covered with a large cribrum). Moreover, the extended depression where the proximal raphe ends are placed (Figs 90, 96, 111) is barely present in D. krammeri (Lange-Bertalot & Reichardt 2000; this study, Figs 129, 130, 133). Diploneis elliptica is another morphologically similar taxon, differing in stria density (8–11/10 μm in D. elliptica vs. 12–15/10 μm in D. stoermeri).

Ecology and Distribution: —M063A, M068A; M077A; M247A; M248A; M272A; M273A; M274A; M276A; M280A; M284A; M287A; M289A; M291A; M329A; M351A: distributed in shallow to deep waters in the northern and southern areas of Lake Hövsgöl on epipelon, epiphyton, epilithon, marl accretions, and sediment habitats.