(Balech) Balech (1973: 27)
Peridinium thulesense Balech (1958a: 92, pl. 6, figs 152–160)
Peridinium conicum f. islandica Braarud (1935: 108, fig. 27)Peridinium sympholis Hermos. & Balech (Hermosilla & Balech, 1969: 9, figs 1–3)
Balech (1976: 56, fig. 39), Dodge (1982: 202, fig. 23L, N), Balech (1988: 105, pl. 39, figs 1–5)
Cells pentagonal to polygonal, 32–40 µm long, 34–40 µm diameter. Thecal surfaces reticulate, with scattered pores; epitheca with slightly concave sides and a small apical horn that is supported by septa along the thecal plate boundaries; hypotheca with an almost flattened base and no apparent antapical spines. Cingulum incised, displaced 0.5–1 cingular width; cingular lists absent. Sulcus narrow and straight, extending posteriorly to antapex, but not into the epitheca; sulcal lists not apparent. Tabulation 1pa, 4', 3a, 7'', 6c, 5''', 1p, 1''''. Plate configuration: metaform quadra.
Type locality, South Sandwich Is. (Balech, 1958a); Southern Ocean, south of Australia; Southern Ocean, near South Georgia (Dodge & Priddle, 1987); bipolar (Balech, 1976).
Protoperidinium is by far the most diverse dinoflagellate genus in Antarctic waters. Approx-imately 40 species have been recorded, most of which are endemic. While a number of taxa, e.g. P. antarcticum, are readily identifiable by their gross morphology, it is usually necessary to obtain good SEM images to determine the tabulation pattern essential for accurate identification. For most species, identification is based on the shape and configuration of the first apical plate (1') and the second intercalary plate (2a). Table 3.1 summarises the plate configuration of Antarctic Protoperidinium species.Balech (1974) recognised three subgenera, Minusculum, Archaeoperidinium and Protoperid-inium. Subgenus Minusculum, with only a single Antarctic species, P. defectum, is characterised by having 6 precingular plates of which 6'' is especially large. Only 2 anterior, intercalary plates are present.Subgenus Archaeoperidinium, represented by two Antarctic species, has 7 precingular plates but only 2 anterior, intercalary plates.Most Antarctic species are referrable to subgenus Protoperidinium which is characterised by 7 precingular plates and 3 anterior, intercalary plates. Primary taxonomic differentiation is based on the shape of the first apical plate, which can be surrounded by 4 (orthoform), 5 (metaform) or 6 (paraform) other plates (Fig. 3.18). The next most significant attribute is the shape of the second anterior, intercalary plate which, once again, can be in contact with 4 (quadra), 5 (penta) or 6 (hexa) surrounding plates (Fig. 3.18). For accurate identification, it is usually essential to determine the form of these two plates, this requiring both a dorsal and ventral view or an apical view. The most common plate configuration among the Antarctic species is metaform quadra (Table 3.1).