Gyrosigma acuminatum

Valves are slender and sigmoid in outline, smoothly tapering to acutely rounded ends. The central area is small and longitudinally elliptical. The raphe is central and straight for half its length, except near distal ends where it follows a sigmoid shape and is slightly eccentric toward each convex side. Proximal raphe ends project into the central area and curve in opposite directions. The axial area is narrow. Areolae are uniseriate and punctate, except near margin where they appear more lineolate. Transverse and longitudinal striae are fine and appear equal in visibility. Transverse striae on either side of central area are very slightly radiate, with the remaining transverse striae parallel to one another, perpendicular to the raphe, and equidistant. Longitudinal striae are equidistant and follow a sigmoid shape, curving slightly toward the valve margin on either side of central area. Smaller specimens appear slightly less sigmoid and more linear-lanceolate.

Gyrosigma acuminatum has been referred to as G. spencerii in instances where valve lengths exceed 140 µm and are very slender. Sterrenburg (1995) compared type material for both species from different regions and reports that longitudinal and transverse stria density, longitudinal/transverse stria ratio, and length and width overlapped. Sterrenburg (1995) concluded that there are no qualitative or ecological differences between the two taxa. Gyrosigma spenceri should now be considered a later subjective synonym to G. acuminatum (Sterrenburg and Underwood, 1997).