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Pterocystidae Cavalier-Smith and von der Heyden, 2007

Diagnosis: Periplast ancestrally with two contrasting types of siliceous scales: tangential basal plate scales (margin not hollow and inrolled but sometimes thickened) oval, sometimes slightly (rarely markedly) narrower at one end, sometimes slightly indented on one side (rarely slightly on both) but never distinctly bilobed; outer bipartite radial scales typically consisting of leaf-like blade with an axial rib, usually extended apically as a projecting needle-like point (blunt or pointed, but unbranched), and lateral wings; radial scale base may be rounded or truncated and may be in the same plane as the blade or somewhat bent away from it (Pterocystis) or markedly extended as a distinct shelf or basal disc (sometimes called basal wing: Mikrjukov, 2002) orthogonal to the main blade (Raineriophrys); species with a pronounced basal wing may have lateral wings that are greatly reduced or absent; if silica scales rarely secondarily missing (Chlamydaster) cell has a distinct mucous coat; funnel-shaped scales absent.
Type genus: Pterocystis.

Included genera:

Pterocystis (eight exclusively and two primarily freshwater spp.).
Raineriophrys (eight freshwater spp.),
Chlamydaster (two spp.; fresh and brackish water),
Echinocystis.

The distinction between Raineriophrys and Pterocystis is not sharp; molecular and ultrastructural investigation of all other Raineriophrys is needed to check whether Raineriophrys is monophyletic and deserves generic distinction.

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