| Summary: | Colonies that could be identified as Thalassiosira rotula, others that could be identified as Thalassiosira gravida and colonies with a mixture of the valve structures of these two species were found coexisting in the same samples from Piedras Coloradas (40°50′46″S, 65°06′18″W). Net samples were collected from surface water at several stations in the northern area of San Matías Gulf in the Río Negro Province, Argentina, from April 1998 to May 2000 and from March 2006 to April 2007. Untreated and cleaned diatom samples were examined using light and scanning electron microscopy. The differential features used to distinguish T. rotula and T. gravida were discussed and refuted with morphological evidence. Based on this analysis, both taxa are assumed to be conspecific by sharing fultoportulae pattern and morphology of the cingulum. Thus, the name of the species must be T. gravida by the principle of priority. A revision of the original material of T. gravida from Baffin Bay was carried out with light microscopy. The original figure 14 was designated as lectotype and the slide no. 64 of the Tempère and Peragallo Collection (2nd edition) curated at the Muséum National d'Histoire Naturelle in Paris was designated as epitype. Fil: Sar, Eugenia Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Ficología; Argentina Fil: Sunesen, Inés. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Ficología; Argentina Fil: Lavigne, Andrea Susana. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Ficología; Argentina Fil: Lofeudo, Soledad. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Ficología; Argentina
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