Valve Ultrastructure, Systematics and Diversity of the Rhopalodiales. I. Introduction and consideration of morphological groups within the genus Epithemia Brébisson ex Kützing.

We conducted a preliminary survey of the valve morphology and ultrastructure of 14 different taxa of the diatom genus Epithemia Brébisson ex Kützing. We documented the structure of the areolae, structure and course of the raphe system, girdle bands with and without septa, and the variation in size and outline of the valves via size diminution across populations from North America, South America and Europe. Based on these preliminary observations, and a comprehensive review of the literature, we classified the genus into five groups, distinguished from one another on the course of the raphe, areolar structure and organization of the frustule. We also provide a review of the genus in terms of nomenclature. This study has been developed to begin comprehensive revisionary studies of the genus.