Cocconeis dirupta W.Greg. (Gregory 1857)

Figs 4, 35–38, Table 1

Cocconeis diaphana W.Sm. var. β (“Sidmouth material”).

Original description

Gregory (1857): 491, 492, pl. 9, fig. 25 reproduced in Fig. 4.

“Broad short oval to orbicular. Length 25.4 to 61 µm; breadth 17.8 to 53.3 µm ”. “Valve thick. Median line irregular, like a slit. Wavy longitudinal striae, except the slit. Fine transverse striae. Appearance of a long stauros, which, under a higher power, disappears as such. The striated surface seems to be ‘torn asunder in the middle’. Vertical striae about 26, transverse striae about 60 in 1 c.d.m.”.

Type material

Lectotype, designated here: slide BM 1420, labelled “Arran 56” in Greville collection, the only syntype found in BM.

Notes

Several localities (Glenshira sand, Mr Miles’s Corallina gathering and several dredgings) cited by Gregory in the original description could contain syntypes, but only the one designated as lectotype was found in BM. According to W. Gregory “ C. diaphana […] which was found occurring with C. dirupta […] may perhaps be an imperfect form of C. dirupta […] or possibly [...] the lower valve’ of the latter”, with some further notes that “it is, however equally probable that these forms belong to different species”.

LM examination of the type material and completed description

Figs 35–38, from BM 1420: n = 16, valves are 18.3–36.5 (20 ± 6) µm long, 17.2–30.8 (26.6 ± 4.7) µm wide; with 13.5–19.5 (16.6 ± 2) SV striae in 10 µm, and 16–23 (18.9 ± 1.9) RV striae in 10 µm. L/l: 1.12 ± 0.06. Valves thick, round-elliptic to almost discoid. Valves rarely found separate, appearing darkbrown. SV with coarse radiate striae, equidistant, composed of transapically oblong areolae arranged in a zig-zag pattern along longitudinal lines, areolae smaller near the margin. One median stria often shorter on one side of the central area (Figs 35–36, arrows). Presence of intercalary short striae near the margin. SV sternum relatively large, fusiform (rarely constricted in its mid-part) with apices often deflected in opposite directions (not illustrated). RV with delicate but clearly identifiable striae, equidistant, strongly bent at the apices, with frequent intercalary short striae near the margin. Raphe almost straight to slightly sigmoid (Fig. 37). RV axial area narrow. Distal raphe fissures close to the margin. Helictoglossae often clearly deflected in opposite directions (Fig. 37, arrowheads); presence of a large and narrow transverse fascia (never reaching the margin, formed by one or two shorter striae, Fig. 38, arrows); proximal raphe endings coarse and relatively close.

The complete frustules appear obscure while the separate RV and SV appear translucent, which probably justifies W. Gregory’s hesitation about the existence of a second taxon (C. diaphana W.Sm.) in his material. On the other hand the R.K. Greville drawing (Gregory 1857: fig. 25) probably superimposed the two valves. Contrarily to the latter drawing (Gregory 1857, fig. 25 reproduced in Fig. 4), the fascia does not reach the valve margin in the many specimens observed in this study.

The slide BM 1420 from Arran 56 (Gregory in Greville collection) is designated here as the lectotype, since nothing else has been traced that could be considered as original material studied by W. Gregory.