Pterocystis Siemensma & Roijackers, 1988 (ref. ID; 4745 original paper)
Actinopoda: Heliozoea (ref. ID; 4745)

[ref. ID; 4745]
Diagnosis; Centrohelid heliozoeans with tangential plate-scales and radial spine-scales; spine-scales bilaterally symmetrical, with a cylindrical shaft and a membranous base which merges into two lateral membranous wings, extending for some distance along the shaft. (ref. ID; 4745)
Type species; Pterocystis pteracantha Siemensma, 1981 (ref. ID; 4745)


Pterocystis anapoda Siemensma & Roijackers, 1988 (ref. ID; 4745 original paper)
Diagnosis; Cell diameter ca. 12 um. The spine-scales are 6.5-10.6 um long. Their shaft is 0.21-0.26 um in diameter, cylindrical, hollow, with a blunt apex. The proximal part of the shaft is strongly bent. The shaft has a membranous base divided in two wings. The base is 2.4-3.0 um long and 1.6-2.5 um wide. Its trapezoid shape resembles a spade or duckfoot. Laterally these wings show a slightly convoluted marginal are which becomes distally broader. The upper surface of the basal membrane is fairly smooth. At the lower surface the shaft is pronounced with a slight indication of radial ribs on both sides of the shaft. Some spine-scale membranes show a slight basal depression. The spine-scales are 3.8-4.1 x 1.8-2.8 um, broadly elliptical to ovoid. There is a central depression with one longitudinal rib and several radial ribs. The surface of the plate-scales is smooth, with a small marginal rim. (ref. ID; 4745)
Comments; Pterocystis anapoda shows some resemblance to Pterocystis pinnata but differs from the latter in the shape of the spine-scales: 1) the apex of P. pinnata is rather sharply pointed while the apex of P. anapoda is abruptly cut off; 2) the basal membrane of P. anapoda is more or less flat while that of P. pinnata shows a deep tapering depression; 3) the wings of P. pinnata extend along the shaft and terminate subapically, whereas the wings of P. anapoda do not extend along the shaft; 4) the shaft of P. pinnata is rather straight whereas that of P. anapoda is strongly bent. (ref. ID; 4745)
Etymology; The specific name refers to the duckfoot-like structure of the spine-scales. (ref. ID; 4745)
Type locality; This species has been collected once in large numbers from a small eutrophic ditch in the northern part of the Netherlands. (ref. ID; 4745)
Pterocystis echinata (Rainer, 1968) Siemensma & Roijackers, 1988 (ref. ID; 4745 redescribed paper)
Emended diagnosis; Spine-scales with a strongly bent shaft, subcentrally attached to a more or less circular membrane. Laterally two small triangular wings connect the basal part of the shaft to the membrane. Both wings extend along the whole shaft as two narrow bands, ornamented with a small number of sharply pointed teeth. The plate-scales are elongately ovoid, 4.7 to 8.0 x 2.2-3.0 um. The upper surface is slightly convex, the lower surface concave with a longitudinal rod-like protrusion. Surface structure smooth. We have found this species in the same material as Acathocystis dresscheri (small moorland pool, pH 5.5). (ref. ID; 4745)
Pterocystis fortesca (Nicholls, 1983) Siemensma & Roijackers, 1988 (ref. ID; 4745 redescribed paper)
Syn; Acanthocystis pantopodeoides Nicholls, 1983 (ref. ID; 4745)
Description; The shape of the spine-base of Acanthocystis pantopodeoides is defined as obovoid, compared with the more triangular shape found with P. fortesca (Nicholls 1983). Commonly we found both kinds of shapes and all intermediate stages within a specimen. From these observations and the measurements we conclude that A. pantopodeoides falls within the range of variation of P. fortesca. Therefore, A. pantopodeoides is a junior synonym of P. fortesca, which name has priority. (ref. ID; 4745)
Pterocystis kilianii (Durrschmidt, 1987) Siemensma & Roijackers, 1988 (ref. ID; 4745 redescribed paper)
Description; The cell diameter of the observed specimens was ca. 14 um. The spine-scales are 7.0-8.4 um long. Their basal membrane is ca. 1.8 um wide. Specimens from the Swedish material had spine-scales with broader wings than the Dutch specimens. Sometimes the edges of the lateral wings were highly wrinkled, though smooth wings were found among wrinkled ones. For the moment, we conclude that lateral folds or wrinkles cannot be used as a taxonomic character. (ref. ID; 4745)
Pterocystis pinnata (Nicholls, 1983) Siemensma & Roijackers, 1988 (ref. ID; 4745 redescribed paper)
Description; Cell diameter ca. 8 um. The spine-scales measured 3.6-5.5 um with the basal membrane ca. 1.8 um broad. The depression formed by the membrane and the basal part of the shaft looks like hollow pyramid with rounded edges. (ref. ID; 4745)
Pterocystis pteracantha (Siemensma, 1981) Siemensma & Roijackers, 1988 (ref. ID; 4745 redescribed paper)
Emended diagnosis; Cell diameter 8-15 um. The spine-scales are 2.5-9.4 um long. The shaft is 0.26 um in diameter, slightly tapering towards the apex. The apex is open and abruptly cut off. The basal part of the shaft merges into an ovoid or nearly circular membrane, 3.0-3.5 um long and 2.4-3.5 um wide; this membrane is ornamented with 50-80 ribs, radiating from the shaft. The base of the shaft bends backwards, forming a distinct depression. The plate-scales are oblong to elliptical, bordered by a 0.1 um thick rim; they are 3.6-5.0 x 2.5-2.8 um, with a large depressed central area. (ref. ID; 4745)
Comments; An emended diagnosis of this species has become necessary since the original diagnosis also includes Pterocystis striata (Nicholls 1983) n. comb. The two species differ from each other as follows: 1) the apex of the spine-scales of P. striata is rounded and closed, whereas the apex of P. pteracantha is open and abruptly cut off; 2) the shaft of the spine-scales of P. striata has its largest diameter distally, tapering strongly proximally, while the shaft of P. pteracantha tapers towards the apex; 3) the number of ribs on the basal membrane of the spine-scales of P. pteracantha is larger and 4) the plate-scales of P. pteracantha are distinctly larger. A comparison of the fine structure of their plate-scales is not yet possible, as the presence of radial structures, revealed by TEM for P. striata, is not detectable in the SEM observations of P. pteracantha. (ref. ID; 4745)
Pterocystis raineri Siemensma & Roijackers, 1988 (ref. ID; 4745 original paper)
Diagnosis; Cell diameter ca. 12 um. The spine-scales are 3.5-7.1 um long. The shaft is cylindrical, straight or curved, slightly tapering towards its closed apex, which bears two teeth. The basal part of the shaft bends strongly and merges into a more or less circular basal plate, which is 0.43-0.86 um in diameter. Laterally this plate is connected to the proximal part of the shaft by means of two membranes, thus forming triangular-shaped hollow, sometimes reduced to a small slit. The plate-scales are oval to slightly ovoid, 2.3-2.8 x 1.3-1.8 um in size, thin and flat without any marginal rim. Their central area is depressed by a longitudinal ridge, ca. 0.08 um broad. No other structures were observed. (ref. ID; 4745)
Comments; This species is very small. The only species which superficially resembles it is Pterocystis erinaceoides n. comb. The latter differs in several aspects from Pterocystis raineri: 1) its plate-scales are ovoid; 2) the apex of the spine-scales terminates in a sharp point; 3) the lateral membranes extend over a fairly long distance along its shaft; 4) the bending of the basal part of the shaft is not so abrupt as in Pterocystis raineri. The size of P. erinaceoides is twice that of P. raineri, though one must allow for a rather large tolerance in size. (ref. ID; 4745)
Etymology; The specific name is given in honour of Dr. H. Rainer. (ref. ID; 4745)