A revision of the monoraphid pennate diatom genus Campyloneis Grunow was carried out based on LM and EM observations. The material examined originated from various herbarium collections and from extant epiphytic diatom communities on leaves of Posidonia spp. We also examined the generitype C. grevillei (Smith) Grunow and the fossil material of C. gheyselinchi Reinhold from which the author extracted the type. Our results clarified the fine structure of C. grevillei and C. gheyselinchi. Of the various varieties of C. grevillei, only the variety argus (Grunow) Cleve was retained. This differs from the nominate variety in the arrangement and shape of the areolae adjacent to the sternum of the araphid valve. The newly described taxon Campyloneis juliae De Stefano differs from all Campyloneis species in areolae ultrastructure and morphology of the valvocopulae. As for the fossil species C. gheyselinchi, the sternum valve areolae are similar to those of C. grevillei, but scarcity of frustules in the type material prohibited evaluation of its variability. For this reason we provisionally maintained its rank of species. The elaborate linking systems among the valvocopulae and valves in Campyloneis species appear to provide structural reinforcement against pressure from neighboring epiphytic diatoms and scouring of seagrass leaves.