Pterocystis plumosa (Dürrschmidt, 1987) Siemensma, 1991
Diagnosis: Periplast, 20-30 µm in diameter (measurement based on whole mount), with two types of scales; details of protoplast unknown. The spine scales are 7-12 µm long, of feather-like appearance. The shaft, 0.01 µm in diameter, is cylindrical, hollow, ending apically with a blunt or abruptly cut off apex. The lateral wings are continued along 2/3 of the shaft, reaching their maximum breadth in the lower half of the spine scale. From here towards the proximal end, they are closely anchylosed with the basal wing and after considerably narrowing, form a patternless hollow stalk. The basal part of the stalk is foot-shaped. The lateral wings are ornamented with thin, delicate ribs which run from the shaft to the outer margin transverselly or obliquely.
Plate scales, 3.0-3.7 x 1.9-2.1 µm, ovoid or elliptical or sometimes asymmetrical. They are thin with deeply concave middle parts, elevated submarginal parts and slightly inflexed margins with a narrow rim. The axial thickening is slightly shifted to the smaller end of the scale.
Remarks: P. plumosa is closely related to P. pulchra. The major distinctive characteristic is the long, wingless distal shaft part. Spine scales with similar foot-shaped bases are also found in one form of P. pulchra which might be intermediary between P. plumosa and genuine P. pulchra.