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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 "r"


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

Raphe - raphe system

One or two slits, or fissures, through the valve face of monoraphid and biraphid diatoms. If two slits are present, each is called a branch of the raphe. Branches may be separated by a silica thickening called the central nodule. Raphe position may be (1) axial, along the apical axis; (2) eccentric, along one margin; or (3) circumferential, around the whole margin of the valve. From Greek for seam.

The raphe is the structure that raphid diatom cells use to move over surfaces.


Raphe ledge

A thin, elevated structure, with the shape of the letter T, that extends along the lateral side of the raphe.

Compared to a conopeum, a raphe ledge is more elevated, more narrow, and does not cover part of the valve face. Common in the genus Amphora and closely related genera.

Rapheless valve - pseudoraphe valve, PRV, central sternum

The valve of a monoraphid pennate diatom lacking a raphe.


The proximal internal terminations of the raphe in a rectangular, elevated, elongate structure. This structure is found in Muelleria, Scoliopleura and Neidium.

Recurved - reflexed

A structure that is bent back on itself.

Rhomboid - rhombus

Having, or approximating, the shape of a rhombus. A rhombus is a parallelogram with all four sides of the same length. Used to describe the shape of a valve (example, Staurosirella rhomboides), a central area, or the girdle view of a frustule (example, Eunotia rhomboidea).

Rimmed depression - hufeisen

A hyaline depression on the inside of a valve. It occurs only in some species of the genus Planothidium and is a depression of the asymmetrical central area on the rapheless valve. See also hood.

Rimoportula - labiate process, jelly pore

A tubular process through the valve of some diatoms. On the internal valve face, the rimoportula opening has the shape of a pair of lips. On the external valve face, the rimoportula opening may be a simple, round aperture at the valve surface or it may be a tube extending out from the valve. Plural form is rimoportulae.

Rimoportulae are found in centric genera such as Stephanodiscus and Aulacoseira; in araphid genera such as Diatoma, Hannaea, Meridion and Tabellaria; and in eunotioid genera such as Actinella, Amphorotia, Eunophora, and Eunotia.

Rimoportulae extrude polysaccharides and other carbon compounds. They are considered to have appeared relatively early in diatom evolution, but to have been lost in a number of more advanced, raphid groups.

Ringleiste - sulcus

An internal, silica ledge that projects into the cell interior from the collum. The ringleiste is found only in the genus Aulacoseira and is variously developed in different species of the genus.


Valve end, or apex, having the shape of a beak, or rostrum. See also capitate.