Nitzschia rigida var. rigidula Peragallo & Peragallo 1900
Homotypic synonym according to GBIF classification (Originally found in sources as accepted taxon within Nitzschia rigida (Kützing) H.Peragallo & M.Peragallo)
Nitzschia sigma f. longissima Manguin 1952
Doubtful according to GBIF classification (Possible variant of Nitzschia sigma f. longissima Grunow, 1881)
Nitzschia sigma f. maxima Halden 1929
Doubtful according to GBIF classification (Possible variant of Nitzschia sigma var. maxima Grunow, 1878)
Nitzschia sigma subsp. curvula (Ehrenberg) Grunow
Homotypic synonym according to GBIF classification (Originally found in sources as accepted taxon within Nitzschia sigma (Kütz.) W.Sm.)
Nitzschia sigma var. rigida Grunow ex Van Heurck 1880
Doubtful according to GBIF classification (Possible variant of Nitzschia sigma var. rigida (Kütz.) Grunow)
Nitzschia sigmoidea var. sigmatella (Gregory)
Homotypic synonym according to GBIF classification (Originally found in sources as accepted taxon within Nitzschia sigmoidea (Nitzsch) W.Sm.)
Stenopterobia sigmatella (Gregory) R. Ross 1986
Homotypic synonym according to GBIF classification (Originally found in sources as synonym of Nitzschia sigmatella var. sigmatella)
? curvula Ehrenberg
Homotypic synonym according to GBIF classification (Originally found in sources as doubtful taxon within Bacillariaceae)
? sigmatella W.Greg.
Homotypic synonym according to GBIF classification (Originally found in sources as doubtful taxon within Bacillariaceae)
