Abstract: During a survey of the diatom flora of some caves on Île de la Possession, the main island of the Crozet archipelago, a small, unusual naviculoid taxon was observed, initially identified as Fallacia lenzii. The new taxon is described as Fallacia emmae but it shows features of two related genera: Fallacia and Germainiella. Thorough morphological research revealed details that, on the one hand, contradicted its placement in Fallacia, such as the relatively low number of poroids on the conopeum, the absence of external canal apertures beside the raphe slit (Fallacia-feature), and striae that are composed of one slit-like areola (Germainiella-feature). The morphology and taxonomy of the new taxon are discussed and questions are raised about the validity of genera recently split off from the genus Navicula on the basis of one, or occasionally two, morphological features.