Thalassiosira allenii H. Takano Thalassiosira allenii Takano (1965): 4, pl. 1: figs. 9–11, pl. 2: fig. 1 — Hasle (1978 a); Hasle and Syvertsen (1997): 51, table 7, pl. 4; Jameson and Hallegraeff (2010): 57, fig. 2.18 B, C; Li et al. (2013): 85, 86, figs. 2–5; Hernández-Becerril et al. (2015); Park et al. (2016); Efimova et al. (2023). Description Valve diameter 17.2 (5–20) µm. Areolae on valve face 22–24 (18–28) in 10 µm in fasciculate pattern, with smaller areolae 30–40 in 10 µm on narrow valve mantle. Central, non-prominent, central process (fultoportula) located next to one, two or three larger areolae. Valve may be covered by minute granules (not observed). Marginal processes long and coarse, 6 (5–9) in 10 µm. External rimoportula, single and prominent, takes place of and is occasionally very slightly inside of the ring of marginal strutted process. Mantle narrow, oblique and angled, 5 (2–3) areolae wide. Diagnosis Thalassiosira allenii differs from T. nordenskioeldii in that the former has a lower mantle in girdle view, a higher areolae density on the valve face and mantle, marginal processes that are closer together and have no collars, processes that are stouter and mantle areolae that are smaller than those in T. nordenskioeldii. Notes iNaturalist ID: 259835781 (Fig. 38).

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