Abstract
The nomenclatural history of the genus Cyanidium (Tilden) Geitler is long and complex, due to uncertainties regarding its taxonomic position. In the course of the last two centuries, Cyanidium has been assigned to Cyanophyta, Chlorophyta, and finally to Rhodophyta (Ott & Sechback 1994, Albertano et al. 2000). On the basis of morphological, ultrastructural and reproductive characters, Merola et al. (1981) instituted two new genera, Galdieria and Cyanidioschyzon, strictly related to Cyanidium, and assigned all three genera to the new class Cyanidiophyceae. Later, in the treatment proposed by Ott & Sechback (1994), all species belonging to Galdieria or Cyanidioschyzon were transferred to Cyanidium. In the most recent taxonomic proposal by Yoon et al. (2014), the tripartite generic architecture has been confirmed, even though a different taxonomic hierarchy was applied at the higher ranks.