Type: — USA. Montana: Spring at base of Square Butte, tributary of Cowboy Creek, Chouteau County, 47.4829 o N, 110.2380 o W, 1400 m elevation, collected from rocks and sediment by Loren Bahls, 22 May 1999. Montana Diatom Collection (MDC) sample 179801; holotype slide MDC B1-22-5, Figs 1–4; isotype slide MDC B1-22-6.
Valves lanceolate to elliptic-lanceolate with subrostrate, broadly rounded apices. Shallow pseudoseptum occupies each apex (Fig. 4). Valve length 47–55 µm; valve width 10.0– 12.3 µm. Raphe filiform, slightly sinuous, with a pronounced bend near the proximal ends, which are weakly inflated. Distal raphe ends hooked towards the same direction. Axial area c. 4 x wider than raphe, following the same contours, including the aforementioned bend. Central stauros broad, largely rectangular, scarcely wider at valve margins. Striae radiate throughout, somewhat curved, concave toward valve center, 19–20 in 10 µm. Areolae in striae 18–20 in 10 µm.
Etymology: —This taxon is named after William Clark (1770–1838), an early 19 th Century Montana explorer (Jones 2004).
Observations: — Stauroneis clarkii was reported by Bahls (2010: 155) as S. subhyperborea Van de Vijver & Lange-Bertalot (2004: 72), from which it may be distinguished by its wider, less protracted apices, shallower pseudosepta, and higher areola density. It differs from S. obtusa Lagerstedt (1873: 36) by having subtly protracted apices and somewhat wider valves relative to their length. Stauroneis microbtusa Reichardt (2004: 442) has more steeply radiate striae and a central stauros that is shaped like a bow tie. Stauroneis clarkii is known only from the type locality, which it shares with S. sacajaweae.