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Abstract
A collection of aerophilous diatoms from northern Madagascar was found to contain two taxa assignable to the modern concept of Orthoseira (but not in the sense of Thwaites). The morphological variability of the valve and cingulum of these two taxa is described and compared. The degree of valve silicification influences spine form and structure, and if such variation is prevalent in other taxa within “Orthoseira”, the use of spine morphology to delimit taxa is questionable. Further, a great deal of confusion exists concerning the nomenclatural status of the genus and its species. Associated with nomenclatural issues, species boundaries within Orthoseira are unclear. Plasticity in morphological features, including degree of silicification, make species identification difficult. The degree of confusion and uncertainty that remain for most of the widely reported species indicate that Orthoseira is in great need of revision.
