Gyrosigma arcuatum (Donkin) Sterrenburg

Pleurosigma arcuatum Donkin (1858): 25, pl. 3, fig. 10.

Gyrosigma fasciola var. arcuatum (Donkin) Cleve (1894): 116 (as ‘ arcuata ’) — Proshkina-Lavrenko (1950): 249, pl. 82, fig. 7; Jin et al. (1985): 80, pl. 24, fig. 188; Cardinal et al. (1989): 18, fig. 7; Stidolph (1994); Sims (1996): pl. 113, fig. 4; Jahn et al. (2005): 309.

Gyrosigma arcuatum — Jahn et al. (2005): 308.

Description

Distinguished by long, rostrate apices curving in opposite directions and sharply turned out from valve body. Valve distinctly sigmoid, lanceolate to elliptical-lanceolate in central portion, narrowing to long and thin extensions that are bluntly rounded or flattened. Valve face fairly flat, length 100.9–105.0 (60–150) µm, width 10.6–14.3 (10–24) µm. Axial area and raphe central and sigmoid, appearing slightly eccentric in extensions of some valves. Central area quite small, orbicular or longitudinally elliptical. Central raphe endings slightly deflected to opposite sides. Longitudinal striae, 25–30 (27) in 10 µm and transverse striae, 24–30 in 10 µm, about equal and distinct. Two marginal, thin plastids, almost exactly opposite.

Notes

These three specimens of Gyrosigma, with long and narrow apical extensions of the valve and curved in opposite directions, are placed into Gyrosigma arcuatum (Donkin) Sterrenburg (Guiry and Guiry 2025), as recommended by Jahn et al. (2005). iNaturalist ID: 254241262, 255788398, 254242421 (Fig. 22).