Cocconeis molesta Kütz. (Kützing 1844)

Figs 1, 15–20, Table 1

Cocconeis molesta var. crucifera Grunow ex Cleve (1895: 174). Type: pl. 30, fig. 23 in Van Heurck (1880 –1885).

Original description

Kützing (1844): 71, pl. 5, fig. 7. 1–2, reproduced in Fig. 1

“ C. minuta, elliptico-oblonga, densissime aggregata, laevissima; limbo nullo. An Callithamnion cruciatum in den Lagunen von Venedig! (Laguna Veneta, Italy) - Länge 1/150–1/140”. – “fig. 1. geglüheten Exemplare, fig. 2. aufgeweichte Exemplare”.

Emended description by Van Heurck

Van Heurck (1896: 290, pl. 29, fig. 823) after observation of the original specimen (F.T. Kützing no. 259 in Van Heurck collection): “valves small, elliptic, finely striate; transverse striae about 40 in 1 c.d.m., reaching to the raphe, formed of puncta, distant and consequently capable of forming longitudinal lines undulate or decussate (according to the direction of the illumination). Raphe robust; central nodule elongated into a narrow acute stauros. Length, 1.5 to 2 c.d.m. Breadth, 1 c.d.m. Marine. North Sea”.

Type material

“An Callithamnion cruciatum in den Lagunen von Venedig ” (Kützing 1844: 71; Kützing 259, Herbarium sheet BM 000905975, BM 18381, holotype); BR slide IX-43-A13 (Van Heurck collection, Kützing 259, isotype).

Notes

Van Heurck (1896) after studying material from F. T. Kützing, emended the description and stated that the observed specimen “corresponds to the var. crucifera of Grunow (H. V. H. Atl.: pl. 20, fig. 20)” (Van Heurck 1880 –1885: pl. 30, fig. 20) and added “it appears to me right to consider it as the type form”.

LM examination of the type material and completed description

Cf. Riaux-Gobin & Compère (2008): figs 56–57; Figs 15–17 from BM 18381, Figs 18–20 from slide IX-43- A 13.

The frustule is small (ca. 16.4 µm long, 9.7 µm wide) and strongly arched (RV concave, SV convex). The RV striation is hardly discernible, with ca. 30 striae in 10 µm (Fig. 18, arrowhead). The SV areolae are arranged along axial rows in a zig-zag pattern (22–23 rows in 10 µm, Figs 16, 17, 20). The helictoglossae are close to the margin and deflected in opposite directions (Fig. 15), the raphe is straight. The SV sternum is narrow and straight, with a small oblong central area. The RV fascia is narrow and half a valve in length.

SEM examination of the Herbarium sheet BM 000905975

The material is very poor and only a broken SV of Cocconeis molesta was observed, with the following morphometrics and features: estimated valve length (> 17 µm), SV stria density (40–42 in 10 µm,

areolae small and transversely oblong), apical SV row of areolae, in a zig-zag pattern (27.5 in 10 µm), SV sternum narrow and thickened in internal view, central area slightly enlarged.