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' O. mobiliensis, apical axis, usually 40-80 /¿m; O. regia, apical axis usually >100 ¿im

' O. mobiliensis, apical axis, usually 40-80 /¿m; O. regia, apical axis usually >100 ¿im

References: Lyngbye, 1819, p. 182, Plate 62, Fig. D; Agardh, 1832, p. 56; Brebisson & Godey, 1838, p. 12; Bailey, 1851, p. 40, Plate 2, Figs. 34 and 35; Schultze, 1858, p. 341; Schultze, 1859, p. 21; Greville, 1859b, p. 163, Plate 8, Fig. 10; Greville, 1866, p. 81, Plate 9, Fig. 16; Grunow, 1884, p. 58; Van Heurck, 1880-1885, Plate 101, Figs. 4-6; Van Heurck, 1909, p. 40, Plate 10, Fig. 141; Karsten, 1905, p. 122, Plate 17, Figs. 2 and 3; Hustedt, 1930, pp. 837, 840, and 846, Figs. 493,495, and 501; Cupp, 1943, pp. 153,154, and 161, Figs. 110-112; Hendey, 1964, pp. 103-105, Plate 20, Figs. 1 and 3, Plate 24, Fig. 6; Drebes, 1974, pp. 85, 90, and 91, Figs. 70,73, and 76; Simonsen, 1974, p. 26; Hoban et al., 1980, p. 594, Figs. 15-38; Hoban, 1983, p. 283; Takano, 1984; Takano, 1990, pp. 300-303.

Generic characters:

Valves elliptical or lanceolate (bipolar).

An elevation (horn) with an ocellus at each pole.

Cells in straight (united by both elevations) or in zigzag chains (united by one elevation). Two or more labiate processes per valve, usually with long external tubes.

Numerous small chloroplasts lying against valve wall.

Characters showing differences between species:

Planktonic species with weakly, littoral with coarsely silicified cell wall.

Curvature of valve face.

Position of processes. Direction of external tubes of processes. Shape and direction of elevations. Valve wall spinose or not.

The presence or absence of resting spore formation.

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