Alveolus

FIGURE 5 Fine structure of the siliceous cell wall.

FIGURE 6 Light micrographs showing areola and process patterns. Scale bars = 10 fim. (a) Coscinodiscus centralis, radial areolation with striae incerted from valve margin, one marginal ring of smaller and two larger (arrows) labiate processes; (b) C. centralis, central rosette of larger areolae, cribra discernible, decussating arcs of areolae; (c) C. centralis, four smaller long-necked labiate processes and one larger process; (d and e) Coscinodiscus radiatus, radial areolation, indistinct decussating rows and fasciculation with striae parallel to the edge row; (f) Thalassiosira punctigera, ribbed margin, one marginal ring of small, densely spaced strutted processes, one larger labiate process (arrow); (g) T. punctigera, bases of occluded processes (arrows), fasciculate areolation with striae parallel to the middle row.

FIGURE 6 Light micrographs showing areola and process patterns. Scale bars = 10 fim. (a) Coscinodiscus centralis, radial areolation with striae incerted from valve margin, one marginal ring of smaller and two larger (arrows) labiate processes; (b) C. centralis, central rosette of larger areolae, cribra discernible, decussating arcs of areolae; (c) C. centralis, four smaller long-necked labiate processes and one larger process; (d and e) Coscinodiscus radiatus, radial areolation, indistinct decussating rows and fasciculation with striae parallel to the edge row; (f) Thalassiosira punctigera, ribbed margin, one marginal ring of small, densely spaced strutted processes, one larger labiate process (arrow); (g) T. punctigera, bases of occluded processes (arrows), fasciculate areolation with striae parallel to the middle row.

Marginal ridge—a ridge between the valve face and the valve mantle, continuous or interrupted, perforated or solid especially in Lithodesmiaceae.

Types of Colonies Separable colonies (von Stosch, 1977)—cells connected by organic substances, separable into smaller units under appropriate conditions (e.g., in Lithodesmiaceae).

Inseparable colonies (von Stosch, 1977)—cells joined by fusion of, or by inseparable interlockings of, silica (e.g., in Skeletonerfia, Chaetoceros, and Cymatosira).

Separation valve (Florin, 1970)—valves where the separation of "inseparable" chains takes place with a structure and process pattern different from that of the other valves.

Chains

Thalassiosira—by threads from strutted processes. Skeletonema—by external parts of strutted processes. Leptocylindrus—by abutting valve faces.

Rhizosolenia—by external part of the single labiate structure (process), contiguous area, and claspers. Eucampia—by bipolar elevations. Cerataulina—by bipolar elevations with spines. Chaetoceros—by setae. Lithodesmiutn—by marginal ridge.

Ribbons

Fragilariopsis—by abutting valve faces. Cymatosira—by marginal linking spines.

Stepped chains

Pseudo-nitzschia—overlapping of cell ends.

Zig zag or star-shaped chains Thalassionema—mucilage pads.

FIGURE 7 Fine structure of the siliceous valve wall and processes as seen with the scanning electron microscope. Scale bars for a, b, and d-g = 1 /xm; for c = 10 /u.m. (a) Thalassiosira sp., inside valve surface with cribra and trifultate strutted process; (b) Thalassiosira sp., inside valve surface with partially broken labiate process; (c and d) Coscitiodiscus spp., central valve region seen from inside valve, internal foramina and external cribra and central unperforated (hyaline) area; (e) Coscitiodiscus sp., marginal part of valve seen from the inside with foramina, six smaller and one larger labiate processes; (f and g) Coscitiodiscus sp., marginal part of valve seen from the inside, foramina, smaller labiate processes and a larger process of a type different from the larger labiate process in 7e.

LABIATE PROCESS = RIMOPORTULA

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