The silica-scaled algae (Synuraceae, Chrysophyceae sensu lato) are compared to other Chrysophyceae, Phaeophyceae and Bacillariophyceae with occasional comparisons to other chlorophyll c-containing algae, scaled protozoa and oomycete fungi. The silica-scaled algae have several unique characters which separate them from the above groups and based upon these differences a new order, Synurales ord. nov., and a new class, Synurophyceae class. nov., are described. The major distinguishing characters of the Synurophyceae class. nov. are: they have chlorophylls a and c1 but lack chlorophyll c2; their flagellar apparatus includes a microtubular root that loops around two parallel flagella and a flagellar root system which occurs in four absolute orientations; the photoreceptor consists of paired flagellar swellings which are not associated with the cell membrane and chloroplast; no eyespot is present; the nuclear envelope is not or is only weakly associated with the chloroplast endoplasmic reticulum. The Synurophyceae class. nov. are about equally distinct from the Chrysophyceae sensu stricto, Phaeophyceae and Bacillariophyceae when the class level characters are compared. Although the Phaeophyceae have a long history of being placed by themselves in the division Phaeophyta, and the Bacillariophyceae and Chrysophyceae have recently been placed alone in the Bacillariophyta and Chrysophyta, respectively, the similarities found among these classes suggest these algae are not so distinct that they require separate divisions. Tentatively, therefore, the Synurophyceae are placed in the division Phaeophyta along with the Bacillariophyceae and Chrysophyceae sensu stricto.
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