Contributor: Pat Kociolek - September 2011
Length Range: 30-200 µm
Width Range: 4.0-13.0 µm
Striae in 10 µm: 19-24
In girdle view, the frustule is distinctly sigmoid. In valve view, depending on the position of the specimen, valves may be distinctly sigmoid. Or, valves may also be linear-lanceolate in the middle of the valve, with the ends curved in opposite directions. The apices are small and capitate. The keel of the raphe is strongly exccentric. Fibula density is 7-12 in 10 µm. Valves lack an expanded central gap in the fibulae.
Basionym: Synedra sigma
Author: Kützing 1844
Length Range: µm
Striae in 10 µm:
Kützing, F.T. (1844). Die kieselschaligen Bacillarien oder Diatomeen. Nordhausen. 152 pp., 30 pls.
Smith, W. (1853). Synopsis of British Diatomaceae. John Van Voorst, London 1853. 89 pp., pls 1-31.