Key to the species of Mylossoma

1. Last abdominal spines clearly separated from anal-fin origin................... M. aureum (Amazon and Orinoco basins)

1'. Last abdominal spines reaching the anal-fin origin or almost so................................................. 2

2. Branched anal-fin rays 31 to 37 (rarely 30)............................. M. albiscopum (Amazon and Orinoco basins)

2’. Branched anal-fin rays 26 to 32.......................................................................... 3

3. Circumpeduncular scales 30 to 34; perforated lateral-line scales 74 to 97; vertebrae 35 to 36................................................................................. M. duriventre (Paraguai, lower Paraná and Uruguai basins)

3'. Circumpeduncular scales 34 to 40; perforated lateral-line scales 95 to 110; vertebrae 37............................................................................................... M. unimaculatum (Tocantins-Araguaia basin)