Key to species of Cnesterodon

1. Minute scales covering lateral and ventral regions of body below pectoral fin, mainly in females (upper portion of the rio Pelotas and rio Canoas of the rio Uruguay drainage, rio Jacuí as well as the headwaters of the rio Maquiné in the Tramandaí system and the headwaters of rio Itajaí - Açu drainage) .............................................. C. brevirostratus

1’. Scales covering lateral and ventral regions of body below pectoral fin approximately of the same size of adjacent scales .................................................................................... 2

2. Caudal peduncle in adult males deep (15.5-17.8 % SL); a prominent dark brown longitudinal band along body sides (rio Cilada, rio Lambari, and rio Guaricanga in upper rio Paranapanema basin, rio Paraná drainage) .... C. hypselurus

2’. Caudal peduncle in adult males low (10.7-15.1 % SL); longitudinal band along body sides lacking or faint.................3

3. Dark brown blotches along body sides circular or irregular......4

3´. Darkbrownblotchesalongbodysidesverticalformingbars....5

4. Six to nine dark brown circular to irregular blotches along body sides; normal size (18.7-24.8 mm [adult males], 20.0- 30.5 mm [adult females]) (rio das Torres, rio Iguaçu basin) ....................................................................... C. omorgmatos

4´. Two to four dark brown circular blotches along body sides; minute size (14.6-17.0 mm [adult males], 17.1-23.2 mm [adult females]) (rio Paraguai and lower rio Paraná drainages) .................................................................................. C. raddai

5. Dark bars on sides of body vertically short, mostly confined to midline, covering less than three scales in a transverse row, never extending to dorsal and ventral profile.............6

5´. Dark bars on sides of body vertically elongate, covering more than four scales in a transverse row, occasionally reaching dorsal and ventral profiles......................................7

6. Post -gonopodium blotch on ventral profile normally developed; bony style at gonopodium tip in adult males comparatively long and slightly arched; membrane on bony style large and progressively narrowing towards tip forming a distal filament (lower rio Uruguay, laguna dos Patos system, río Negro, río Salado, western drainages of Argentina and small coastal drainages of Uruguay and Argentina) ........................................... C. decemmaculatus

6’ Post -gonopodium blotch on ventral profile large; bony style at gonopodium tip in adult males comparatively short and very arched; membrane on bony style never forming a distal filament (rio Ribeira de Iguape drainage) .... C. iguape

7. Twenty -five or 26 caudal-fin rays; 28 to 31 scales on longitudinal series (upper portions of rio Iguaçu and its upper tributaries) ....................................................... C. carnegiei

7’ Twenty to 22 caudal-fin rays; 25 or 26 scales on longitudinal series (upper rioAraguaia drainage) ............ C. septentrionalis