Acanthocystis nichollsi  Siemensma and Roijackers, 1988

This name was proposed for a species identified by Nicholls (1983) as Acanthocystis pectinata. Nicholls (1983) gave an emended diagnosis of A. pectinata, but his specimens differ from the original diagnosis of this species. A. pectinata is characterised by the presence of numerous short spine-scales, all of the same length (Penard 1889, 1904) and a scanty number of long spine-scales (Rainer 1968). Rainer emphasised that the presence of a very small number of long spine-scales is a specific character, which was confirmed by Siemensma (1981). The Canadian specimens deviate clearly from the European specimens in having many long spine-scales, and Nicholls (1983) observed all intermediate lengths between long and short spine-scales. Therefore, the light microscopic appearance of these specimens does not correspond well with the original description of A. pectinata nor with the emendation by Rainer (1968). The surface of the plate-scales of the Canadian specimens is smooth, whereas the specimens I observed from several localities throughout the Netherlands, and which I consider to be identical to A. pectinata, all have granulated plate-scales. In TEM, the plate-scales of A. nichollsi have radial slits, which are lacking in A. pectinata. Acanthocystis nichollsi closely resembles A. polymorpha Dürrschmidt, 1985, the plate-scales of which lack any granules or slits.