Valve ultrastructure of Didymosphenia dentata (Bacillariophyta): an endemic diatom species from Lake Baikal

Valve ultrastructure of the diatom Didymosphenia dentata, an endemic specie s from Lake Baikal, Russia, is presented. This species has a valve exterior with occluded areolae, prominent marginal spines, dilated proximal raphe e nds and distal ends that are deflected at 90 degrees angles in the same dir ection. Ridges or flanges spiral around each spine. Stigmata are lacking. I nternally the central nodule, raphe sternum and helictoglossae are prominen t. D. dendata differs From Didymosphenia geminata in valve shape, presence of marginal spines and lack of stigmata. Recent suggestions that the two sp ecies are conspecific are not supported by observations presented here.