Cavinula scutiformis Mann & Stickle

Cavinula scutiformis (Grunow ex A. Schmidt) Mann & Stickle (Figs 107–111)

Basionym: Navicula scutiformis Grunow in Schmidt et al. 1881, fig. 70: 62

Valves elliptic, with rounded apices. Valve length 23–32 µm, width 17–22 µm. Striae radiate throughout valve length. Striae composed of fine, elliptic to somewhat rounded areolae, 12–14 in 10 µm. Stria density, 18–24 to 28–30 in 10 µm. Central area narrow, linearlanceolate, transforming into a defined central space transversally elongated, rounded to elliptical in shape. Central area formed by gradual shortening of striae along both valve margins. Raphe system linear; proximal raphe fissures tear-drop like. Distal raphe fissures pore-like and small, terminating on valve face.

Type:— Stavanger, Norway. Grunow in Schmidt et al. 1881, fig. 70: 62 (holotype).

Observations:— This taxon is identified by the elliptic valve shape, large transversely elongated central area, high stria and areola densities at the apices, and the presence of distinct pore-like fissures at the valve apices.

Distribution:— C. scutiformis occurs in the northern regions of Europe, Asia and North America. In the CANA collection this taxon was identified from 35 localities in western (excluding Saskatchewan, Manitoba) and eastern Canada, but not from the tundra region north of 60 degrees. In the framework of this study, C. scutiformis was identified in six more localities from Canada, in which it was previously not recorded: sediment core C2 (50–51 cm) recovered from Flower Round Lake, Crosson Lake Oakley (sediment core) Muskoka District, Clear Lake Oakley (sediment core), Muskoka District, (Ontario) and additionally Big Moose Lake, Moriah Parch Pond (New York) and Walden Pond (Massachusetts) in the United States. This taxon was also confirmed in Lac aux Araignees (Québec). C. scutiformis was identified in samples with low to circumneutral pH (5.1–7), an average weighted mean conductivity of 34.5 µS/cm, with a range of 26–65 µS/cm, and an average weighted mean alkalinity of 62 µeq/L, and a range between 6 and 153 µeq/L.

Distribution Records:— Grunow in Schmidt et al. (1881, Atlas der Diatomaceen-kunde, Band I, Serie I-III, fig. 70: 62), Krammer & Lange-Bertalot (1986, fig. 59: 10, 11, Europe), Lange-Bertalot & Metzeltin (1996, fig. 24: 13, 14, fig. 114: 3, 4, Europe), Foged (1971, fig. 15: 20, Alaska).