Family Acanthocystidae Claus, 1874, emend. Cavalier-Smith & Von der Heyden, 2007
Diagnosis: Periplast with two or three types of siliceous scales, a basal layer of typically oval plate scales (usually without an inrolled hollow edge; only exception Raphidocystis) and an outer layer of radial funnel-shaped siliceous scales and/or radial needle-like scales sometimes apically branched and/or with a peltate radially symmetrical basal disc lacking a lateral indentation (unlike Choanocystis radial scales). Exclusively freshwater, except perhaps occasionally for R. tubifera.
Type genus Acanthocystis.
Acanthocystis (18 spp. mostly freshwater; 2 marine).