Cavinula variostriata Mann

Cavinula variostriata (Krasske) Mann in Round et al. 1990 (Figs 123, 124)

Basionym: Navicula variostriata Krasske 1923, 197: 12

Valves linear, with rounded apices. Valve length 30–32 µm, width 7–8 µm. Striae curved and radiate throughout valve length. Stria density 24–27 in 10 µm. Axial area narrow and linear. Central area rectangular to broadly rounded, extending to valve margin with reduced marginal striae. Central area forms a large internally expanded nodule. Raphe linear; proximal raphe fissures externally small teardrop like; internally proximal fissures weakly deflected in opposite directions and terminal fissures ends with small helictoglossae. Distal raphe fissures externally deflected in opposite directions and terminate on valve face. Areolae round to elliptic externally and internally. Internal areolae openings covered by hymen.

Type:— Hessen, Germany.

Observations:— This taxon is identified by the linear valve margins, large central area, evident internally expanded central nodule, and the oppositely deflected terminal raphe fissure that do not extend down the mantle.

Distribution:— Cavinula variostriata occurs in the northern regions of Europe, and North America. In the CANA collection this taxon was identified from 26 localities from Axel Heiberg Island, Ellesmere Island, Baffin Island (Arctic Archipelago), Lac Karinbou (northern Québec) and unnamed lakes in central Ontario. In the USA, C. variostriata was tentatively reported from 13 locations but to date only samples from Murder Creek (New Jersey), Red Mill Pond (Delaware) and Johnson Creek (Montana) have been verified. Specimens from New Jersey and Delaware are more elliptical in shape and the central area is smaller compared to specimens from the Arctic Archipelago. C. variostriata needs a more detailed study in North America.

Distribution Records:— Lange-Bertalot et al. (1996, fig. 18: 27–30, Europe), Krammer & Lange-Bertalot (1986, fig. 59: 10, 11, Europe), Güttinger (1991, fig. 2.05: 31–34, Great Britain), Foged (1974, fig. 13: 8, 9, Iceland), Potapova (2014, fig. 214, Bering Island, Kamchatka Peninsula, Russia).