Key To Genera

la. Pili present 2

lb. Pili absent 4

2a. Cells curved in broad girdle view 3

2b. Cells straight in broad girdle view Plagiogrammopsis, p. 183

3 a. Pilus valves concave Arcocellulus, p. 179

3b. Pilus valves convex or convex in the middle and concave closer to the elevations Minutocellus, p. 182

4a. Fascia present, linking spines absent Brockmanniella, p. 179

4b. Fascia absent, linking spines present Cymatosira, p. 181

Genus Arcocellulus Hasle, von Stosch, 8c Syvertsen 1983 (Plate 36, Table 42)

Type: Arcocellulus mammifer Hasle, von Stosch, 8c Syvertsen. References: Hasle et al., 1983, p. 54, Text Figs. 10 and 11, Figs. 272-333 and 408-421.

Generic characters:

Solitary or in loose ribbons.

Valve outline narrowly lanceolate, elliptical to subcircular, smaller valves oblong to subcircular, often without pili.

No marginal linking spines.

Marginal ridge inconspicuous.

Cell wall weakly silicified; structure not resolved with LM. Tubular process.

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