Stauroneis anceps

Valves are lanceolate to linear-lanceolate. The apices are protracted and narrowly rostrate. The axial area is moderately wide and linear, expanding slightly near the central area. The central area (in Northern Rockies specimens) is a narrow rectangular stauros, hardly expanded toward valve margins and sometimes with shortened striae. The raphe is lateral. Proximal raphe ends are weakly expanded, weakly deflected, and widely spaced (they do not extend into the central area). Striae are radiate throughout. Areolae are variably spaced and number 20-28 in 10 µm.

Stauroneis anceps is one of the most commonly misidentified Stauroneis species in North America. Reichardt (1995) designated the lectotype for this species (Tafel 16, fig. 1) from a population (Sippe 1) in Ehrenberg's type material from Cayenne, French Guyana. Similar forms present in Ehrenberg's type material (Sippe 2 and Sippe 3) were later described as Stauroneis reichardtii (Lange-Bertalot et al. 2003). Specimens of S. anceps from the Northern Rockies tend to be somewhat larger, have a lower stria density and a smaller stauros than specimens of Sippe 1 in the type population, but they are otherwise quite similar. Until further work is completed, we consider the population of S. anceps described here to represent this species as described by Reichardt (1995) from Ehrenberg's type material.