Type:— USA. Montana: Blodgett Lake, Selway-Bitterroot Wilderness, Ravalli County, 46.2505 o N, 114.4544 o W, 2072 m elevation, collected from rocks and sediment by U.S. Forest Service personnel on 10 September 1991. MDC sample 110901; holotype slide MDC P3-4-1, Figs 5–6.
Valves narrow lanceolate to linear-lanceolate with gradually attenuated subrostrate apices. Moderately deep pseudoseptum occupies each apex (Fig. 6). Valve length 25–38 µm; valve width 6.1-7.9 µm. Raphe filiform, nearly straight with somewhat inflated proximal ends. Distal raphe ends hooked in same direction. Axial area linear c. 3 x wider than raphe, widening somewhat near central area. Central stauros broad, shaped like a bow tie. Striae strongly radiate throughout, 22–26 in 10 µm. Areolae in striae 24-26 in 10 µm.
Etymology:—This species is named after Meriwether Lewis (1774–1809), an early 19 th Century Montana explorer (Danisi 2009).
Observations:— Stauroneis lewisii was reported by Bahls (2010: 121) as S. pseudoschimanskii Van de Vijver & Lange-Bertalot (2004: 57), which has elliptic-lanceolate valves with short, broadly subrostrate apices. Besides the type locality, Stauroneis lewisii has been collected from two other small lakes in southwestern Montana: Fred Burr Lake, Selway-Bitterroot Wilderness, Ravalli County, 46.3291 o N, 114.4163 o W; and Bloody Dick Pond #6, Beaverhead County, 45.1119 o N, 113.4447 o W.