H.A.Thomsen (1976: 41, figs 24–28, 29 (holotype), 30–33)
Buck & Garrison (1983: 1264, fig. 22), Thomsen et al. (1990: 378, figs 49–57)
Cells solitary. Lorica funnel-shaped, to 18–20 µm long, 18–30 µm diameter; longitudinal costae 4; transverse costae 2. Longitudinal costae each comprising 3 costal strips; base of lorica chamber constructed of 4–6 costal strips, of which 4 are an extension of the longitudinal costae; posterior costal strip abuts the second most posterior strip for c. 25–33% of their length. Anteriormost transverse costa comprising 8 costal strips; anterior tips of the anterior costal strips swollen, bifurcated and joined at the centre of every other strip in the anterior transverse costa, these strips abut every other intersection between strips in the posterior transverse costa. Some specimens with other transverse costae comprised of 6 or 7 costal strips. Protoplast obovoid, 10 µm long, 5 µm diameter, located in base of lorica chamber, suspended by a sheath from the second most posterior costal strip.
Inshore and oceanic stations of Prydz Bay, East Antarctica; Lützow-Holm Bay (Takahashi, 1981, as “species 9”), Weddell Sea (Buck & Garrison, 1983); Denmark (Thomsen, 1976).
While there are differences between the Danish type material of P. circularis and material collected in Antarctica, we do not consider the differences significant enough to warrant the recognition of a new species at this time. The type specimen of P. circularis has 8 costal strips in the anterior costae and only 6 in the posterior costae. Prydz Bay specimens are similar in costal strip morphology to those from Lützow-Holm Bay and the Weddell Sea. Thomsen (1976) discussed the differences between P. circularis and the closely allied P. quadricostata Throndsen from Denmark.