Hemidiscus cuneiformis var. ventricosa (Castracane) Hustedt Figs 41, 42

COSCINODISCALES Round & Crawford: Hemidiscaceae Hendey

Ref. illus.: Hustedt 1927 –1930, fig. 542d; Hendey 1964, pl. 22, fig. 9; Ricard 1987, figs 30–34; Stidolph et al. 2012, pl. 13, fig. 71 and pl. 4, figs 104, 107; Álvarez-Blanco & Blanco 2014, pl. 11, fig. 6; the images in Round et al. 1990, pp. 192–193, are also evidently this species.

Samples: GU66F-4

Dimensions: long axis 71–74 µm, short axis 41–45 µm

Diagnostics: Distinctively shaped, lens-shaped in valve view but the frustules wedge-shaped (cuneiform) because the valve faces are at an angle to one another. Areola pattern similar to Actinocyclus, to which it is related, but with a series of radiating patterns around the margin, especially on the less strongly curved side. Large rimoportulae visible in the oblique view (Fig. 42). A pseudonodulus is present on the flatter side (Fig. 41).

Comments: Two valves observed in one sample. The ventral curvatures of H. cuneiformis Wallich vary from straight to sinuous and Hustedt combined several older Eudotia species as varieties. Images shown by Hendey (1964), Stidolph et al. (2012, pl. 13, fig. 71) and identified simply as H. cuneiformis Wallich, have a ventral outline as shown here; Hendey did not comment on varieties; Stidolph et al. (2012, pl. 10, fig. 30) show the nominate variety, again without noting the variety. The species differs further in valve shape from two fossil species (Harwood & Maruyama 1992). Although Round et al. (1990) (reprinted in AlgaeBase) said that the valves are cuneiform, it seems to me that Wallich’s (1860) epithet refers to the shape of the frustule (described by Round et al. 1990 as “shaped like orange segments”), rather than to the shape of the valve.