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An annulus is a hyaline ring on the valve face of radial centric and some bipolar and multipolar centric diatoms. The plural is annuli.
This ring is the first silica formed during valve morphogenesis and if visible or well developed (depending on the genus) may not be visible in later stages of cell development.
In some bipolar centric groups (e.g., the Ardissoniales) the annulus lies near the valve margin. During morphogenesis, the annulus produces striae on both sides apically and lengthens the striae on both sides transapically.
The annulus is homologous to the central nodule in pennate diatoms.