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Pterocystis pteracantha
  

Pterocystis pteracantha – after Siemensma, 1991

Pterocystis pteracantha   (Siemensma, 1981) Siemensma & Roijackers, 1988
  
 Diagnosis: Cell diameter 8-15 µm. The spine-scales 2.5-9.4 µm long; shaft 0.26 µm in diameter, slightly tapering towards the apex which is open and abruptly cut off. Basal part of the shaft merges into an ovoid or nearly circular membrane, 3.0-3.5 µm long and 2.4-3.5 µm wide, ornamented with 50-80 ribs, radiating from the shaft; base of the shaft bends backwards, forming a distinct depression. Plate-scales oblong to elliptical, bordered by a 0.1 µm thick rim, 3.6-5.0 X 2.5-2.8 µm, with a large, depressed central area.

Remarks: This species differs from Pterocystis striata (Nicholls, 1983) 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.
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