Lacustriella concinna Rodionova, Pomazkina & Sherbakova sp. nov.

Light microscopy (LM): (Figs 1–15). Valves linearly lanceolate to elliptically lanceolate with slightly protracted rostrate apices. Valve length 37.0–82.5 μm, width 16–24 μm. External axial area narrow at apices gradually expanding into irregularly oval central area. Raphe filiform with straight central endings, terminal ends bent oppositely. Striae uniseriate parallel or slightly radial become radial towards apices, 12–13 in 10 μm. One longitudinal canal along each margin.

Scanning electron microscopy (SEM): (Figs 16–23). Externally, areolae in striae round and open, irregular in the central part of the valve, 13–17 in 10 μm (Figs 17, 18). Areolae bordering the longitudinal canals significantly elongated transapically, slit-like, open (Fig. 18). Areolae encircling the axial and central areas enlarged. The surface of longitudinal canals is covered with uni- or biseriate rows, continuing the striae, and consisting of 1–5 small pores (Figs 18–20). Areolae on the mantle transapically elongated similar to those at the border with the longitudinal canals. Proximal raphe endings lay in slightly elongated drop-shaped depressions (Fig. 17). Distal raphe endings curved to opposite sides. Internally, areolae in striae are covered with hymenes, and raphe endings terminate on helictoglossae, central helictoglossae merged (Figs 22, 23). The cingulum consists of two bands with a row of pores. At the same apex, the open band alternates with the closed one (Figs 19, 20).

Type: RUSSIA. Lake Baikal: Northern Baikal, Olkhon isl., Cape Khoboi (53.576111 ºN, 107.8075 ºE), littoral zone at 12 m, sand sample collected by Sherbakova T.A. on June 18, 2021 (holotype: NSK0201989 housed at the Central Siberian Botanical Garden, Novosibirsk, Russia, M.G. Popov Herbarium NSK. Representative type specimen is illustrated in Fig. 2. Isotypes: slide 0145–ВК, Limnological Institute SB RAS, Irkutsk, Russia, station 40, near the city of Baikalsk).

Etymology: the specific epithet concinna means harmonious, proportionate.