Abstract
Electron microscope (EM) is essential for the morphological description of small taxa, but also reveals taxa hitherto grouped under the same name although presenting marked ultrastructural differences: e.g., Cocconeis scutellum var. parva. The latter taxon has been interpreted differently by authors, while no type has been examined since the original description. A slide from the Grunow’s collection in W, annotated by Albert Grunow as containing C. scutellum var. parva was examined and detailed for the first time. This slide allowed for a more precise definition of C. scutellum var. parva and is here designated as lectotype. This paper analyzes the history of C. scutellum var. parva and attempts to exclude definitions that are too far from the original drawings and from the lectotype illustration. A tropical–population from Rodrigues Island (Indian Ocean) composed of small specimens with stria densities significantly denser than observed on the nominate variety, is here described as an epitype of C. scutellum var. parva and is illustrated with SEM. A small taxon from Tahiti Island (South Pacific Ocean), seemingly close to C. scutellum var. parva, has unique raphe valve valvocopula with cupuliform papillae. The latter specificity is considered a simple variation and may illustrate the range of morphological variability admitted for a taxon.