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Diagnosis. Arius species can be distinguished from all other ariid genera by the following combination of non-exclusive characters: (1) frontal bones without laminar mesial projection (shared with Amphiarius, Aspistor, Cathorops, Cephalocassis, Cryptarius, Doiichthys, Galeichthys, Hemiarius, Nedystoma, Nemapteryx, Osteogeneiosus, Potamarius and Potamosilurus latirostris); (2) orbitosphenoid lateral projections moderately long, its lateral edge irregularly shaped (shared with Carlarius parkii); (3) mesopterygoid subtriangular and very long [shared with Amphiarius rugispinis, Bagre (with exception of B. panamensis), Cathorops, Cephalocassis, Cinetodus, Cryptarius, Doiichthys, Genidens, Hemiarius, Nedystoma, Nemapteryx, Pachyula, Potamarius, Sciades assimilis, S. leptaspis, S. guatemalensis, S. platypogon, S. sagor and S. seemanni]; (4) third basibranchial cup shaped its anterior portion very wide, posterior portion elongated [shared with Amphiarius, Aspistor, Batrachocephalus, Cephalocassis borneensis, Cinetodus, Cryptarius, Hemiarius stormii, Netuma, Pachyula, Plicofollis (with exception of P. platystomus and P. tenuispinis) and Sciades (with exception of S. bonillai, S. guatemalensis, S. proops, S. passany and S. sagor)]. Supplementary morphological characters. Cephalic shield granulated visible under the skin; fenestra of moderate size limited by frontal and lateral ethmoid bones visible under the skin; medial groove of neurocranium very distinct, limited by frontal bones and / or on supraoccipital; posterior cranial fontanel moderately large, long and narrow; fenestra limited by supraoccipital, pterotic and sphenotic bones absent; fossa limited by pterotic, supracleithrum and extrascapular relatively large; epioccipital not invading into dorsal portion of cephalic shield; occipital process triangular, moderately long and wide, progressively narrower toward its posterior part; anterior and median nuchal plates fused, indistinct forming a structure of semi-lunar asp