Velum

Velum is a general term for a thin, porous layer of silica occluding an areola. The pores are at least 30 nm in diameter and often not circular. The plural is vela. Cribrum, rota, and vola are each a type of velum. Scanning electron microscopy is usually needed to resolve the structure.

In a loculate areola, the velum may form the external or internal wall opposite the foramen, depending on the taxon. A velum may be compound, combining a cribrum with a hymen or a cribrum with a vola.

Also see occlusion, which is a general term for covering.

Citations

Anonymous (1975) Proposals for standardization of diatom terminology and diagnoses Nova Hedwigia, Beihefte 53: 323-354
Cox, E.J. (2004) Pore occlusions in raphid diatoms - a reassessment of their structure and terminology, with particular reference to members of the Cymbellales. Diatom 20: 33-46.

Publication Link: 10.11464/diatom1985.20.0_33
Cox, E.J. (2012) Ontogeny, homology, and terminology - wall morphogenesis as an aid to character recognition and character state definition for pennate diatom systematics Journal of Phycology 48: 1–31

Publication Link: 10.1111/j.1529-8817.2011.01081.x
Ross, R., Cox, E.J., Karayeva, N.I., Mann, D.G., Paddock, T.B.B., Simonsen, R. and Sims, P.A. (1979) An amended terminology for the siliceous components of the diatom cell Nova Hedwigia, Beihefte 64: 513-533
Round, F.E., Crawford, R.M. and Mann, D.G. (1990) The Diatoms. Biology and Morphology of the Genera. Cambridge University Press, Cambridge, 747 pp.