Polyplacocystis Mikrjukov, 1996 (ref. ID; 4884 original paper)

[ref. ID; 4884]
Diagnosis; Centrohelid heliozoa with a periplast comprising one or several types of spindle-form or flat siliceous elements (scales). The latter are placed in mainly tangential direction and have reticular or smooth texture of the upper surface. There are no internal septa in scales. Radial elements in the periplast are absent. A single stalked member is known; a mucous stalk is supported by very oblong scales. Five species. (ref. ID; 4884)
Remarks; The genus includes P. symmetrica and P. ambigua with a reticular texture of the scale surface; P. ambigua differs by the shape differentiation of scales from internal layers of the periplast towards the external ones (Nicholls & Durrschmidt 1985; Siemensma & Roijackers 1988). Another part of the genus consists of three species with a smooth scale surface. The fresh-water species P. pallida [whose spindle-form scales have tubular ends and widening (unwrapped) central parts] and the marine species P. pedunculata [whose cell body has a long mucous stalk supported by very oblong narrow scales] stand out from other observed members of the group. All other isolated specimens reported from both fresh and marine waters (Siemensma 1981; Bardele, cit. acc. Margulis 1981; Nicholls & Durrschmidt 1985; Mikrjukov 1993, 1994) ought surely to be regarded as P. marginata. Despite of remarkable variety in the scale shape among ever observed isolates of P. marginata (especially in length/width-ratio), there is a whole spectrum of transitional steps from classical P. marginata (see Siemensma & Roijackers 1988) up to organisms reported by Bardele (cit. acc. Margulis 1981) as R. neapolitana. Scales representing two extreme positions in the transitional row are interesting to belong to fresh-water organisms, while scales of marine and brackish-water ones lie at the middle. However, organisms with the broadest scales were isolated from soft stagnant waters in Estonia and northern Russia, while Fig. 12 demonstrates the narrowest scales belonging to organisms from very hard-water lakes in Crimea [and in Italy, Bardele (1981)]. Similar polymorphism of the shape of plate scales, which surely depends on the hardness of water, was recorded in the acanthocystid centroheliozoon, Choanocystis aculeata Hertwig & Lesser by Mikrjukov (1995). Many "old" species (which were not described by EM) may be synonyms of R. marginata, for example, R. bruni Penard, R. coerulea Penard and R. infestans (Wetzel). Recently described periplast elements of R. marina Ostenfeld (iv-group) are undoubtedly more primitive in comparison with the just mentioned groups (i) and (ii). Its "scales" could originate in evolution by the silicification of tangential organic spicules similar to those in Sphaerastrum. We propose to regard R. marina as a single member of a new genus Parasphaerastrum. (ref. ID; 4884)
Type species; Polyplacocystis symmetrica (Penard, 1904) (ref. ID; 4884)