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Abstract
Material chosen as the lectotype of Amphora holsatica Hustedt and others of the type series has been examined using light and electron microscopy, and a more comprehensive concept of A. holsatica was established through an expansion of its description. In light of this new description comparisons with the related A. pseudoholsatica Nagumo & Kobayasi and A. holsaticoides Nagumo & Kobayasi were conducted. Although specimens described as A. pseudoholsatica show slight differences from A. holsatica in morphometric data, the morphology and the slope of the mantle, as well as the presence of a flap-like rib at the junction of the mantle and valve face in the former, allow us to delimit these two taxa. The specimens described as A. holsaticoides differ from A. holsatica in length and in stria density. They also exhibit subtle differences in morphology of the ventral striae, slope of the dorsal mantle and morphology of the central area, conopeum and raphe slit. These differences are considered sufficient to justify the maintenance of A. holsaticoides. A. subholsatica Krammer was also included in this analysis because the author compared it with A. holsatica in the protologue of the species. Morphometric parameters and valve morphology of A. subholsatica are, however, more similar to those of A. acutiuscula Kützing, with which it may be conspecific.