Type material of the diatom species Eunotia arcuoides Foged, housed at the ANSP Diatom Herbarium and at the Natural History Museum of Denmark, is investigated with light and electron microscopy, and a lectotype specimen of this species is designated. Eunotia arcuoides is a poorly known species that shares a number of morphological characters with a few Eunotia taxa known from alkaline waters. Among other characters of this species, we describe an unusual “epithemioid” or “amphoroid” shape of the frustule and a unique round helictoglossa. Eunotia arcuoides has equally wide dorsal and ventral parts of the mantle, and the valve faces in recently formed frustules are parallel The girdle bands are, however, wider in their dorsal parts compared to ventral parts, so that the cells become progressively more amphoroid as they grow and more girdle bands are added.