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Diatoms of the United States is now known as Diatoms of North America.
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Glossary terms starting with "c"


A valve end, or apex, having the shape of a head, or the shape of a rounded knob.

Other terms for valve ends include subcapitate (less strongly capitate), rostrate, and subrostrate (less strongly rostrate). Subcapitate and subrostrate are quite similar and many taxa may be described as having subcapitate to subrostrate valve ends.

In general, capitate and subcapitate ends are more expanded than the most narrow point on the valve, while rostrate and subrostrate ends are not more expanded than the most narrow point.


A type of central process unique to the genus Orthoseira. The internal openings are simple, while the external openings are composed of well-defined collars.

Central nodule

In raphid diatoms, the thickly silicified area located between the proximal raphe slits. Often thickened, both in valve and girdle views. It is the site where the silica deposition vesicle deposits the first silica during valve formation.


Central tube

An internal tubular expression of the fultoportula


The series of linking, siliceous bands associated with a valve. Each band is called a girdle band, or copula (from Latin for band or link; plural is copulae). Plural form is cingula.

Each valve has its own cingulum and the valves of most diatoms have a cingulum. The epivalve has the epicingulum and the hypovalve has the hypocingulum. The epicingulum always overlaps the hypocingulum. The two cingula together compose the girdle, or cincture.

Girdle bands may be secreted after the vegetative cell division that produced the valve. In other words, while the deposition of the valve is tied to cell division, the deposition of its girdle bands may not be.


Having the shape of a club. Usually applied to describe the asymmetry of Gomphoneis and Gomphonema in valve view.



A valve surface covered with small bumps or elevated bits of silica.


The narrow, hyaline area on the valve mantle in the genus Aulacoseira. A small furrow, or sulcus, separates the collum from the portion of the valve mantle with areolae. See also ringleiste.


A thin flap of silica on the external valve face, lying along the apical axis. Hyaline or finely porous. Proximal margin formed by the edge of the raphe slit. Extends unsupported over the valve face. Distal margin is free. May lie flat or may be slightly to distinctly elevated. May partially or completely cover the striae. Found in genera such as Fallacia, Lyrella, Mastogloia, Microcostatus, Nitzschia and Sellaphora. Plural form is conopea. From Greek for canopy.

Details of the conopea are best viewed with the scanning electron microscope, but may be visible with the light microscope as longitudinal lines near the raphe or as unornamented areas of the valve face.

Conopea have been found to contain symbiotic nitrogen-fixing cyanobacteria.

See also pseudoconopeum.


Striae that lean toward the central nodule, with the sternum as the origin. Convergent striae may occur throughout the valve, at the valve center, or at the valve apices. In contrast, striae are radiate, or divergent, when they lean away from the central nodule, with the sternum as the origin.


An unornamented, elongated thickening of the valve. Usually refers to a thickening parallel to the striae. Plural form is costae. From Latin for rib.

Morphogenetically, a costa parallel to the striae is a secondarily thickened virga.

See also axial costa.

Costa (primary, secondary, tertiary)

In genera such as Diatoma, Meridion, and Tetracyclus, costae can be qualified by the extent to which they traverse the valve face. Primary costae fully traverse the valve. Secondary costae start at the valve margin and end at the sternum. Tertiary costae start at the margin and end before reaching the sternum.

See also costa.



A ridge of silica on the side of the satellite pore away from the central tube.


An internal valve occurring in species in the genus Craticula. This craticular valve consists of a raphe-sternum and robust transverse bars. Frustules with normal morphology produce the craticular valve under conditions of high solute concentration. The formation of these internal valves is not linked to mitosis.

Cribrum - sieve plate, velum

A thin, domed, porous plate of silica covering the internal opening of an areola, found in genera such as Thalassiosira and Stephanodiscus. A cribrum is a type of velum. Visible only with the SEM. Plural form is cribra.

A similar structure with finer pores, found in many raphid diatoms, is the hymen.


Having the shape of a cross. Usually applied to describe the valve outline of some diatoms.



Having the shape of a wedge. For example, valves of Surirella amphioxys have cuneate apices. The genera Gomphoneis and Gomphonema have species with frustules that are cuneate in girdle view.