Genus Glaucostegus Bonaparte, 1846 Type species. Rhina cemiculus Geoffroy St. Hilaire, 1827 (subsequent designation). Description. Nostrils moderately elongate to very elongate and narrowly separated, length usually greatly exceeding internasal distance; posterior aperture very elongate and narrow, not or only slightly oblique to anterior aperture. Anterior nasal flaps bilobed, not fused to form a nasal curtain, inner lobe short based; inserted on snout adjacent nostril anterior margin, lateral and well anterior to innermost corner of posterior aperture (flap insertions very widely separated, by much more than internasal distance); anterior aperture very large, elongate oval, posteromedial edge formed by narrow outer lobe-like extension of anterior nasal flap (positioned medially on nostril rather than near its lateral corner); outer lobe preceded by long anterior fold; inner lobe of anterior nasal flap narrow with straight to weakly convex margin; posterior margin of anterior aperture bordered by medium-sized (almost as wide as aperture), long-based, posterolateral nasal flap; free lobe of posterolateral nasal flap extending posteromedially, but rarely reaching middle of posterior aperture of nostril; posterior nasal flap weak, width similar to posterolateral nasal flap and base short (much less than half of nostril length). Lower jaw straight or weakly undulate; skin margin forming upper lip variably concave; 3–5 short lateral grooves at its angle; oral groove weak, confined to chin. Species. G. ce m i c u l u s (Geoffroy St. Hilaire, 1817), G. granulatus (Cuvier, 1829), G. h a l a v i (Forsskål, 1775), G. obtusus (Müller & Henle, 1841), G. thouin (Anonymous, 1798), G. t y p u s (Bennett, 1830).

Published as part of Last, Peter R., Séret, Bernard & Naylor, Gavin J. P., 2016, A new species of guitarfish, Rhinobatos borneensis sp. nov. with a redefinition of the family-level classification in the order Rhinopristiformes (Chondrichthyes: Batoidea), pp. 451-475 in Zootaxa 4117 (4) on pages 472-473, DOI: 10.11646/zootaxa.4117.4.1, http://zenodo.org/record/271659