Pterocystis raineri  Siemensma and Roijackers, 1988

Diagnosis: Cell diameter ca. 12 µm. Spine-scales 3.5-7.1 µm long, shaft cylindrical, straight or curved, slightly tapering towards its closed apex, which bears two teeth; basal part of the shaft bends strongly and merges into a more or less circular basal plate, which is 0.43-0.86 µm in diameter. Laterally this plate is connected to the proximal part of the shaft by means of two membranes, thus forming a 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 µm in size, thin and flat without any marginal rim; central area depressed by a longitudinal ridge, ca. 0.08 µm broad. No other structures were observed.

Remarks: This species is very small. The only species which superficially resembles it is Pterocystis erinaceoides. 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.
5. the size of P. erinaceoides is twice that of P. raineri, though one must allow for a rather large tolerance in size.

The specific name was given in honour of Dr. H. Rainer.