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We present here an ultrastructure-based revision of the diatom genus Anorthoneis using both light (LM) and electron microscopy (SEM). The original slides and type material of all known Anorthoneis species were analysed and new, previously unseen, morphological features of the frustules, such as rota-like vela and ligulate structures around the valve margin, were revealed. The phylogenetic relationships among Anorthoneis taxa were investigated based on morphological characters using a maximum parsimony method. This analysis indicated a large, well-supported clade that included taxa that exhibited similarities in external and internal details of the central areas in both raphe-sternum and sternum valves. Revealed synapomorphies within the genus include externally and internally transversely dilated central area on both sides of the raphe-sternum valve, externally transversely dilated central area on both sides of the sternum valve, sternum valve striae parallel at the center and radiate at the apices, and areolae with hymenes in both valves. During a survey a new species Anorthoneis arthus-bertrandii sp. nov., was found on seagrass leaves collected from Siladen Island (Indonesia). This taxon is sister to A. hyalina, but can be easily differentiated from the latter by smaller valve dimensions, central area size and shape, and rota-like vela occluding sternum valve areolae - a feature that has never been observed in other Anorthoneis taxa

Type slides and the last remnant of the type sample of Haslea wawrikae were examined in LM and SEM. An emended diagnosis is presented, one new finding being that the transverse bars of the basal layer on either side of the raphe are offset—the bars on the one side aligning with the areolar foramina on the other. The species is confirmed (SEM) to have a fully developed raphe and the valve structure characterising Haslea species: an internal “grate-like” basal layer and an external tegumental layer with continuous longitudinal fissures. An erroneous identification of H. wawrikae was traced in the literature and the existence of similar, but non-conspecific, species is suspected.

Seven new Haslea species demonstrate that the genus has evolved a richer morphological repertoire than so far reported. H. feriarum sp. nov. has a valve contour suggesting an Amphora; H. staurosigmoidea sp. nov. has a sigmoid valve with a pseudostauros; H. tsukamotoi sp. nov. and the closely similar H. meteorou sp. nov. possess uniquely shaped external central raphe endings and a fully sideways-tilted internal raphe system; H. clevei sp. nov. and H. avium sp. nov. also have central raphe ending shapes not yet described in the genus; H. amicorum shows interrupted longitudinal fissures besides the continuous fissures characteristic of Haslea. A survey of the Haslea species described, for differentiation of our taxa, led to two conclusions: 1) the data in the protologue of H. indica Desikachary & Prema are spurious and 2) “Navicula” duerrenbergiana Hustedt is here transferred to the genus Haslea. The basic Haslea morphology is a sandwich-type valve, with a grate-like inner layer (here called basal layer) and an outer layer (here called tegumental layer) perforated by continuous longitudinal fissures. These two layers are shored by upright longitudinal “bulkheads”, here called saepes, seen to be perforated in the valves that permitted their observation. This morphology is closely similar to that of Gyrosigma (and Pleurosigma). The great variety of central external raphe ending patterns in Haslea, Gyrosigma and Pleurosigma is shown and discussed.

Gyrosigma tenuissimum (W.Sm.) J.W.Griffith & Henfr. was examined in the original material and an emended description is presented. Its protologue contains an error of taxonomic significance: in the type material, the valve and raphe sternum do not show the considerable flexure described and illustrated in Smith (1853). A comparison with Gyrosigma coelophilum N. Okamoto & Nagumo revealed that the latter, although similar in several characters, differs sufficiently in others to warrant separate specific status. Descriptions of two new taxa are given: Gyrosigma tenuissimum var. gundulae var. nov. and Gyrosigma baculum sp. nov. Both show very fine longitudinal striae at or beyond the limit of ordinary light microscopy and are practically non-sigmoid. © 2014 Magnolia Press.