Anorthoneis excentrica (Donkin) Grunow 1867: 10

Anorthoneis excentrica (Donkin) Grunow 1867: 10 ( Figs 1–13 View FIGURES 1–9 View FIGURES 10–13 )

Basionym:— Cocconeis excentrica Donkin (1858), p. 25 , pl. 3, fig.11.

References:— Grunow (1867), pl. 1, 1a, 2–11; De Toni (1891), p. 468–69; Cleve (1895), p. 166, pl. 3, fig. 12; Peragallo & Peragallo (1897 –1908), p. 24, pl. 5, fig. 1; Boyer (1916), pl. 16, figs 30, 31; Boyer (1927), p. 241; Hustedt (1933), p. 319, fig. 780; Van der

Werff & Huls (1957), pl. 14, fig. 93; Hendey (1964), p. 182, pl. 36, fig. 10; Sterrenburg (1988), p. 363–376, figs 5–6, 24–27, 29; Witkowski et al. (2000), p. 97, pl. 42, fig. 20, pl. 54, figs 9, 10.

Description. Valves broadly elliptical to almost circular in outline ( Figs 1, 3 View FIGURES 1–9 ). Apical axis: 32–35 μm, transapical axis: 25–28 μm.

SV: — Externally, valve face ( Fig. 10 View FIGURES 10–13 ) flat, becoming convex towards the undifferentiated, wide mantle, terminating in small marginal area ( Fig. 11 View FIGURES 10–13 ). Eccentric sternum not well discernible in SEM ( Fig. 10 View FIGURES 10–13 arrow), not reaching the valve margin, terminating in small, roundish hyaline area ( Fig. 10 View FIGURES 10–13 arrowhead). Transapical striae uniseriate and radiate on both the valve face and the mantle, 11–12 in 10 μm close to the sternum, becoming denser towards the margin ( 12–13 in 10 μm), composed of circular areolae (approx. 250 μm, 10–11 in 10 μm) externally occluded by hymenes with short, radial perforations ( Fig. 11 View FIGURES 10–13 ). In the valve margin, close to the end of each stria, a smaller imperforated areola is present ( Fig. 11 View FIGURES 10–13 arrows).

RSV: — Externally, valve face flat to convex changing abruptly to concave in the mantle. Asymmetric raphe-sternum developed entirely within the valve face lacks a marked axial area. Proximal raphe endings expanded, pore-like, converge coaxially toward a broad central nodule ( Fig. 12 View FIGURES 10–13 ). Distal raphe endings similar in shape to the proximal ones, terminating in roundish terminal areas distant from the valve margin ( Fig. 12 View FIGURES 10–13 ). Radial striae uniseriate, becoming biseriate in the mantle, 10–13 in 10 μm, composed of small circular areolae ( 12–16 in 10 μm) occluded by hymenes ultrastructurally similar to those in the SV. One smaller areola present at the end of each stria ( Fig. 12 View FIGURES 10–13 arrows). Internally, both the raphe-sternum and the central nodule thickened and prominent. Distal raphe endings terminating in reduced helictoglossae hardly reaching the hyaline terminal areas ( Fig. 13 View FIGURES 10–13 ).