Crista marginalis

A crista marginalis is the low, marginal ridge of silica between the valve face and the valve mantle on the raphe valve of some genera in the Order Achnanthales. From Latin for "marginal crest". Plural is cristae marginalis.

Examples include some species in the monoraphid genera Cocconeis, Planothidium, and Bennettella.

Crista marginalis has become a useful term for working with these monoraphid genera but has not had a formal definition. Romero and Rivera (1996) first used the term but did not define it. They cited Ross et al. (1979) as an origin but that paper defined only the broader term "marginal ridge" which applied to several groups: centrics, raphids, and mediophytes. The definition provided here is an explicit description of this feature.

Citations

Romero, O.E. and Rivera, P. (1996) Morphology and taxonomy of three varieties of Cocconeis costata and C. pinnata (Bacillariophyceae) with considerations of Pleuroneis. Diatom Research 11(2): 317-343.
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

Bennettella ceticola external distal raphe end 1 — External view of distal raphe end of *Bennettella ceticola* (SEM). White arrows mark the crista marginalis. Scale bar = 1 µm.