Type: — USA. Montana: Upper Wolverine Lake (Fig. 25), Ten Lakes Scenic Area, Kootenai National Forest, Lincoln County, 48.9705 o N, 114.9221 o W, 1890 m elevation, collected from rocks and sediment by Loren Bahls, 9 September 1990. MDC sample 108601; holotype slide MDC P3-3-1, Figs 22–24.
Valves lanceolate with rounded, unprotracted apices. A deep pseudoseptum occupies each apex (Fig. 24). Valve length 88–105 µm; valve width 16.3–17.5 µm. Raphe lateral with straight proximal ends, slightly inflated. Distal raphe ends hooked to same side. Axial area narrowly lanceolate, widens gradually towards central area. Central stauros nearly rectangular and placed somewhat diagonally to apical axis. Striae radiate throughout, 17–18 in 10µm. Areolae in striae 12–15 in 10 µm.
Etymology:— Stauroneis thompsonii is named for David Thompson (1770–1857), an early 19 th Century Montana explorer and cartographer (Jenish 2003).
Observations: — Stauroneis thompsonii was reported by Bahls (2010: 61) as S. aff. catharinae Van de Vijver & Lange-Bertalot (2004: 27). However, it has significantly wider valves and lower stria and areola densities than S. catharinae. It is known only from the type locality, a small lake in the Galton Range in northwest Montana (Fig. 25).