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' Measured with the light microscope. ' No. in parentheses occasionally found.

References: Grunow, 1879, p. 688; Castracane, 1886, p. 169, Plate 14, Fig. 5; Rattray, 1890a; Hustedt, 1930, p. 374, Fig. 196; Wiseman & Hendey, 1953, p. 49, Plates 1 and 2; Kolbe, 1957, p. 33, Figs. 5 and 6, Plate 4, Figs. 46-49; Round, 1980; Rivera et al., 1989; Round et al., 1990, p. 206.

Ethmodiscus rex and E. gazellae are the only two of the 15 taxa recognized by VanLandingham (1969) that are regularly referred to. They are usually regarded as the largest single celled members of the marine phytoplankton having the shape of large boxes, with approximately equally sized diameters and pervalvar axes.

Generic characters:

Cell wall weakly silicified.

Valve areolae small, distinctly separated, in radial rows outside a central area.

Each theca with one wide band with wide hyaline margins and areolae in straight parallel rows. Processes (revealed as labiate processes with SEM) in central part of the valve.

Heterovalvy, one strongly convex valve and the other with flattened valve face and a broad and flat mantle (Rivera et al., 1989).

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