Palmeria, p. Ill
2a. Pervalvar axis high, cell diameter up to 2 mm . . Ethmodiscus, p. 110 2b. Pervalvar axis and diameter smaller Coscinodiscus, p. 98
Genus Coscinodiscus Ehrenberg 1839 emend. Hasle 8c Sims 1986 Lectotype: Coscinodiscus argus Ehrenberg (proposed by Ross 8c Sims, 1973, conservation proposed by Fryxell, 1978b).
References: Ehrenberg, 1839, p. 128; Fryxell, 1978b, p. 122; Hasle 8c Sims, 1986a, p. 316.
Coscinodiscus is usually regarded as one of the largest marine planktonic diatom genera [400-500 validly described taxa (VanLandingham, 1968)]. A great number of the most frequently recorded Coscinodiscus species have been transferred to Thalassiosira, Azpeitia, and Actinocyclus, or to new genera as illustrated by the fact that of the approximately 20 Coscinodiscus species recorded in the Arctic literature between 1853 and 1911 only 4 are now regarded as belonging to the genus.
The Coscinodiscus species dealt with in this chapter are frequently recorded in the literature and/or they have been critically examined with LM and EM. Some of them are readily distinguished by special, pronounced morphological features, whereas others are easily confused. Our interpretation of the species is based on the often incomplete original diagnoses, the descriptions in Hustedt (1930), as well as recent LM and EM investigations referred to under the various species. It should be noted that the data on cell diameter of these species vary considerably in the literature. We have tried to use a compilation of data, and the sources are not given specifically in all cases.
All the species dealt with here have (see Fig. 13):
Radial areola pattern.
Two larger marginal labiate processes (macro-rimoportulae).
Marginal ring of smaller labiate processes.
Numerous, usually disc-shaped, chloroplasts.
Comments on terminology (Figs. 6a-6e, 7c-7g, 10, and 13)
The radial rows of areolae (striae) may be grouped into more or less distinct bundles (fasciculation), and/or spiraling rows (decussating arcs) may be present (Figs. 6a-6e and 10b and 10c). Radial rows inserted from the margin (inserted striae, incomplete striae) are necessarily present in this type of areola pattern (Figs. 6d and 6e and 13). The bundles are separated by more or less distinct unperforated radial areas (wide interstriae, hyaline spaces, hyaline lines), on valve face, as well as on valve mantle (Plate 16). Valve center is usually occupied by either a hyaline (nonareolate) area or by a rosette of larger areolae (Figs. 6b, 7c and 7d, and 13). The structure of this central area may vary parallel to the cell diminution by the the vegetative multiplication (Schmid, 1990).
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