A ring of costal thickness, often surrounding one or more processes and with a structure different from that of the rest of the valve.
A thickened silica ring on the valve face from which silica ribs radiate in centric diatoms.
References
Barber, H.G. & Haworth, E.Y. 1981. A guide to the morphology of the diatom frustule. Freshwater Biological Association, Ambleside, Cumbria, UK. 112 pp.
Hasle, G.R. & Syvertsen, E.E., 1996. Marine Diatoms. In C.R. Tomas (ed), Identifying Marine Diatoms and Dinoflagellates. Academic Press, Inc., California. pp. 5-385.
Round, F.E., Crawford, R.M. & Mann, D.G., 1990. The Diatoms: Biology and morphology of the genera. Cambridge: Cambridge University Press. 747 pp.
