Pterocystis vietnamica Prokina, Radaykina, Mylnikov, 2020
Diagnosis: Details of the protoplast structure are unknown. Cell covered by leaflike spine- and ovoid
plate-scales. Spine-scales consist of hollow cylindrical shaft, basal and lateral wings. Shaft weakly developed, 1.52-2.29 µm long, 0.08 µm in diameter. Lateral wings extend along shaft, distally narrowing to the pointed end, connect with basal wing at their base, forming large spherical subapical protrusion 0.30-0.46 µm in diameter. Lateral wings 0.84-1.19 µm in the widest part, with transverse slits; edges have thickened margin (0.06–0.07 µm in diameter). Plate-scales 1.67-3.08 × 1.35-2.24 µm with central depression and slightly thickened margin (0.07-0.08 µm in diameter).
Ecology: Freshwater. In detritus and sand under stone, Dong Nai River, Vietnam.
Remarks: This species is similar to P. foliacea in the leaf-shaped spine-scales but, unlike P. vietnamica, P. foliacea has a thin “stalk” formed in the basal part instead of a quite large spherical protrusion. Moreover, the lateral wings of P. vietnamica have transverse slits. The small rounded basal protrusion of the basal wing is characteristic of Pseudoraphidocystis veliformis (Dürrschmidt 1985) Mikrjukov 1997; however, the lateral wings of the latter are much wider and the scale is almost rounded in outline, the lateral wings bear radially diverging ribs instead of slits, and the subapical protrusion is weakly developed. The well-developed basal protrusion, passing into the broad hollow stalk, is characteristic of the species Pterocystis pteracantha (Siemensma 1981) Siemensma et Roijackers 1988 and P. pulchra (Dürrschmidt 1985) Siemensma 1991, but these species are distinguished from P. vietnamica by the almost funnel-like shape of the spine-scales, the presence of radial ribs on the lateral wings, and the well-formed hollow stalk having a flattened base. Longitudinal slits on the lateral wings are typical of the species Raineriophrys pteromorphos (Dürrschmidt 1987) Mikrjukov 2001, which, nevertheless, has other distinctive traits as well: (1) the lateral wings do not extend along the entire shaft length; (2) the shaft is well developed and ends with a truncated, rather than tapered, distal end; (3) the lack of basal protrusion.