McFadden, Moestrup & Wetherbee (1982: 107, fig. 19)
van den Hoff et al. (1989: 448, figs 2–19); Davidson & Marchant (1992b), McMinn & Hodgson (1993: 936, fig. 4)
Cells subcylindrical to conical, to 8–18 µm long, 7–9 µm diameter. Body scales of 3 types: an outer rather loose layer of crown scales, and an inner layer of closely fitting box scales and smaller footprint scales. Crown scales c. 300 nm across the base, c. 300 nm high, with 4 centrally connected arches. Box scales square, c. 300 nm on each side, with perforated walls c. 80 nm high. Footprint scales positioned among the box scales, c. 80 nm long. Flagella thin, marginally longer than the cell body, with an outer layer of limulus scales and underlying rows of pentagonal scales; limulus scales 400–500 nm long, with irregular perforations; pentagonal scales c. 60 nm diameter, with a central boss. Cells with a single lobed chloroplast, with 1 parietal pyrenoid opposite a pair of eyespots.
Coastal waters and fast-ice of the Vestfold Hills, East Antarctica; Southern Ocean, south of Australia.
Sym & Pienaar (1993) discussed the diagnostic characteristics of the green flagellate Pyramimonas. Cells have either 4 or 8 flagella of equal length emerging from the anterior end of the cell. Up to 7 types of non-mineralised scales cover the cell body and the flagella, and these scales are diagnostic at the species level.
Cyst morphology and ultrastructure have been described in detail by van den Hoff et al. (1989). They also documented the developmental changes that occur during the production of cysts from vegetative cells.