The diatom genus Tetracyclus Ralfs (Bacillariophyta) is composed largely of extinct, freshwater species many of which have been used as stratigraphic markers across several continents under the assumption that they are relatively widespread and confined to precise geological epochs. Until recently the taxonomy of the genus relied almost exclusively on the shape and dimensions of the preserved siliceous valves. This study forms part of a revision of the entire genus. In this paper the morphology of fossils from the `ellipticus' species group is discussed. Significant to this study is the relative usefulness of diatom valve dimensions and overall shape, seen here in the context of siliceous morphogenesis and the appearance of particular valve `shapes' at particular stages in their life cycles. In addition, alternative ways of representing character data have been applied to establish whether the `ellipticus' species group is monophyletic with respect to the genus. Results suggest that the `ellipticus' species group is not monophyletic and that the elliptical shape of valves is better viewed as a variable property of ontogeny.