Key to species of Cynopotamus

1a. Dorsal-fin rays ii,10 (Paraguay, lower Paraná and Uruguay river basins) ................................ C. argenteus

1b. Dorsal-fin rays ii,9 (very rarely ii,10).......................................................................................................... 2

2a. Perforated lateral-line scales 85–90; horizontal scale rows from dorsal-fin origin to lateral line 17–18 (upper rio Juruena, Mato Grosso, Brazil) ................................................................................... C. juruenae

2b. Perforated lateral line scales 91–125; horizontal scale rows from dorsal-fin origin to lateral line 20–31 .. 3

3a. Horizontal scale rows from anal-fin origin to lateral line 17–21................................................................. 4

3b. Horizontal scale rows from anal-fin origin to lateral line 22–30................................................................. 5

4a. Developed gill-rakers on first branchial arch 8–9; branched anal-fin rays 36–43 (rivers of Guyana, French Guiana and Surinam and rio Branco basin, Roraima and Araguari basin, Amapá, Brazil) C. essequibensis

4b. Developed gill-rakers on first branchial arch 10–11; branched anal-fin rays 39–45 (rio Xingu basin, Mato Grosso and Pará, Brazil) ............................................................................................................. C. xinguano

5a. Horizontal scale rows from dorsal-fin origin to lateral line 23–25 (rio Tocantins and rio Araguaia basins, Goiás and Pará, Brazil) ......................................................................................................... C. tocantinensis

5b. Horizontal scale rows from dorsal-fin origin to lateral line 25–31.............................................................. 6

6a. Perforated lateral line scales 125 or more; branched anal-fin rays 49–51 (rio Magdalena and rio Cauca basins, Colombia) C. magdalenae

6b. Perforated lateral-line scales 98–121; branched anal-fin rays 36–49 .......................................................... 7

7a. Branched anal-fin rays 47–49 (río Orinoco basin, Venezuela) ............................................... C. bipunctatus

7b. Branched anal-fin rays 36–47...................................................................................................................... 8

8a. Horizontal scale rows from dorsal-fin origin to lateral line 29–31; branched anal-fin rays 35–38 (rios Tocantins and Araguaia basins) ................................................................................................. C. gouldingi

8b. Horizontal scale rows from dorsal-fin origin to lateral line 25–29; branched anal-fin rays 36–47............. 9

9a. Branched anal-fin rays 36–39 (Peruvian Amazon and tributaries of rio Solimões in Brazil) ... C. amazonus

9b. Branched anal-fin rays 39–47.................................................................................................................... 10

10a.Perforated lateral-line scales 98–109; branched pectoral-fin rays 13–15; branched anal-fin rays 39–44 (rios Paraguay and lower Paraná basins, Brazil) ......................................................................... C. kincaidi

10b.Perforated lateral-line scales 110–121; branched pectoral-fin rays 15–17; branched anal-fin rays 40–47 .. ................................................................................................................................................................... 11

11a.Horizontal scale rows from anal-fin origin to lateral line 29–31; branched anal-fin rays 45–47 (río Atrato basin, Colombia) ...................................................................................................................... C. atratoensis

11b.Horizontal scale rows from anal-fin origin to lateral line 25–28; branched anal-fin rays 40–46 (Maracaibo basin, Venezuela) ..................................................................................................................... C. venezuelae