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Pterocystis clarkii  Cavalier-Smith and Von der Heyden, 2007
 
Diagnosis: radial scales (3-5 µm) with broad central rib tapering at both ends, distally lightly curved to one side, and extending apically well beyond the lateral vanes, both as in P. pyriformis, P. paliformis, P. pinnata and P. canadensis, but shape of lateral vanes radically different from those species: basally completely truncated or even mildly indented and with distinct lateral lobes giving it a trefoil appearance somewhat as in P. anapoda (but anapoda has very different not tapered, straight axial rib that does not reach to base of scale); lateral wings unribbed.
Broadly oval plate scales (2-4 µm), very slightly indented on one side, with indistinct slightly curved axial ridge and no furrows.

Ecology: Eutrophic freshwaters. Type locality Priest Pot (), Cumbria, UK.