PLATE 16 Coscinodiscus concinnus: girdle view. Broader interstriae distinct, one marginal ring of processes at ends of interstriae. Coscinodiscus wailesii: girdle view. High, steep valve mantle with two rings of labiate processes, one ring in the junction zone between valve face and mantle, and one ring close to valve margin at end of wide interstriae. Scale bar = 20 fim.

The two larger processes (Figs. 7e-7g) are usually observed with LM, in the larger species in water mounts, and in most species in acid cleaned material mounted in a medium of a high refractive index (Figs. 6a and 6c and 13). When noticed in the past, they were termed "unsymmetrische Prozesse" (Hustedt, 1930), or "asymmetrical processes or apiculi" (Cupp, 1943; Hendey, 1964). In some species the internal part and the shape of the'processes are not revealed with LM; their position may be revealed, however, by indentations of the valve margin. When discernible with LM, the smaller labiate processes (Figs. 6a and 6c and 13) were termed in the past "Randdornen" (Hustedt, 1930) or "spinulae" (Cupp, 1943; Hendey, 1964). Smaller labiate processes may be present in a second marginal ring or on the valve face, usually associated with wide interstriae, or at the points of origin of incomplete striae.

"Interstitialmaschen—interstitial meshes" (Hustedt, 1930; Cupp, 1943) are located at the points of origin of incomplete striae and are identical with labiate processes, small pentagonal areolae or larger adjacent areolae, or unper-forated areas.

Characters showing differences between species (partly based on Fryxell & Ashworth, 1988):

Cell shape in girdle view.

Cell diameter.

Valve shape.

Areolae (size) in 10 fim near center and peripheral.

Height of mantle, measured as number of areolae.

Areola pattern on valve face.

Central area, shape (rosette of larger areolae or nonareolated = hyaline).

The presence/absence of hyaline lines (interspaces, wide interstriae).

Shape of the larger processes.

Distance between the larger processes.

Distance between the smaller marginal processes (measured as number of areolae).

The presence or absence of processes on valve face.

Chloroplast outline.

See Schmid (1990) for further details on variation of some of these characters through the life cycle.

The species dealt with in this chapter are grouped according to the shape of the frustules in girdle view with the same qualifications as made for Thalassio-sira spp.

C. jonesianus

C. concinniformis

C. jonesianus

C. concinniformis

C. granii

C. bouvet

PLATE 17 Coscinodiscus concinniformis: girdle view with several columns of segments. Cosci-nodiscus jonesianus: girdle view with evident outer parts of larger labiate processes. Coscinodiscus granii: girdle view, showing valve mantle and bands of uneven width. Coscinodiscus bouvet: girdle view, short chain. Scale bar = 20 jum.

A. Frustules discoid to cylindrical: C. alboranii, C. argus, C.

asteromphalus, C. centralis, C. concinniformis, C. concinnus, C. jonesianus, C. marginatus, and C. wailesii (Tables 20a and 20b).

B. Frustules coin shaped: C. radiatus (Tables 20a and 20b).

C. Frustules with a stepped elevated valve face: C. bouvet (Tables 20a and 20b).

D. Frustules wedge shaped: C. gratiii (Tables 20a and 20b).

A. Frustules discoid.

Coscinodiscus argus Ehrenberg (Plate 15, Tables 20a and 20b) References: Ehrenberg, 1839, p. 129; Ehrenberg, 1841a, p. 145; Ehrenberg, 1854, Plate 21, Fig. 2, Plate 22, Fig. 5; Hustedt, 1930, p. 422, Fig. 226; Hasle & Sims, 1986a, Figs. 1-7, 33, and 34. Girdle view: Valve face flat or slightly depressed in the center; rounded margins; valve mantle steep and high.

Valve view: Small central hyaline area just discernible with LM; areolae in the center differ slightly from the others in shape, but not in size. Decussating arcs present but not prominent. Hyaline lines (ribs) visible internally (SEM) but not perceived with LM, neither are the marginal processes. Foramina and cribra just discernible with LM. Distinctive features: Areolae increase in size from the center of the valve toward the middle of the radius, then decrease in size toward the margin.

TABLE 20a Morphometric Data of Coscinodiscus spp.

Diameter Valve areolae No. of Valvocopula

Species (p.m) in 10 |xm bands/theca width (ftm)

Group A

Diameter Valve areolae No. of Valvocopula

Species (p.m) in 10 |xm bands/theca width (ftm)

Group A

C. alboranii

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