Cribrum

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.

Alternative Terms

sieve plate
velum
Yellowstone Vcd 026
Image Credit: Sarah Spaulding
Scanning electron micrograph of the inside of a valve of Stephanodiscus yellowstonensis showing highly magnified, domed cribra covering the areolae. Scale bar = 1 µm.
Stephanodiscus Domed Cribra
Image Credit: Sarah Spaulding
Scanning electron micrograph of the inside of a valve of Stephanodiscus showing a less magnified view than the above image of domed cribra covering the areolae. The arrow indicates a single cribrum. Central fultoportulae, six of them, are also visible.