Isthmia enervis Ehrenberg Isthmia enervis Ehrenberg (1838): 209, pl. 16, fig. 6 — Kützing (1844); Smith (1856): pl. 47, fig. F; Van Heurck (1881): XCVI, figs. 1–3; Hendey (1964): 110, 111, pl. XXV, fig. 3, pl. XXV, fig. 2; Round (1984); Round et al. (1990); Sims (1996); Guiry and Guiry (2025). Description Cells form branching colonies with large mucilage pads. Cells rhomboid-shaped, heteropolar. Valves elliptical and ovoid, apical axis 70 (48–200) µm, length of whole frustule in girdle view 195–268 (150–300) µm. No transition between valve face and mantle. Single pseudocellus per cell, with cluster of smaller areolae. Externally, areolae are large, cribrate, suspended by struts, ovoid to quadrate and enclosed by robust silica. In the apical direction, every 2–3 areolae have a pore in the silica mesh framework; in the transapical direction, 1–2 areolae between poroids. Areolae pattern curved, slightly radiate transapically, 2–3 in 10 µm. Areolae size is mostly regular over valve face, becoming smaller at pseudocellus. Girdle bands complete, with smaller areolae than valve and closed by cribra. Notes iNaturalist ID: 204882487 (Fig. 24).

Published as part of Webber, Mark, Humphrey, Elaine, van Asselt, Arjan, Chang, Alice, Morien, Evan & Simon, Andrew D. F., 2026, Diatoms (Bacillariophyta) of the Salish Sea, Northeast Pacific: annotated checklist and new species reports, pp. e 189060 in Biodiversity Data Journal 14 on page e189060, DOI: 10.3897/BDJ.14.e189060