Notes on the diatom collection of the Natural History Museum, London (BM) VII: An account of some original specimens of Pyrgodiscus (Bacillariophyta) with notes on Stephanogonia (Bacillariophyta)

Specimens of the diatom genus Pyrgodiscus held at the BM have been examined and two species are recognised and accepted: Pyrgodiscus armatus and Pyrgodiscus simplex. Both are extinct fossil species but known only from light microscope data. The sole defining character recognised (so far) is two series of spines: (1) one series of spines surrounding the base of the valve, (2) the other series surrounding the central pillar. The status of the genus, however, is unclear as they may be resting spores. The classification of Pyrgodiscus is uncertain. Although a relationship with Stephanogonia was suggested some time ago, the only option at this time is to leave Pyrgodiscus as incertae sedis and to encourage investigation and characterisation of higher taxa.