Dichotomous key of the species from Brazilian waters

1. Complete ribs covering the entire shell flank ................................................................................... 2 – Complete ribs restricted to posterior half of the shell flank .............................................................. 3

2. Anterodorsal margin descending ...................................................................................................... 4 – Anterodorsal margin forming a shoulder .......................................................................................... 5

3. Rostrum short ........................................................................................................... Cardiomya sp. 3 – Rostrum very short ........................................................................................................................... 8

4. Numerous ribs (~ 20); equidistant ....................................... Cardiomya cleryana (d’Orbigny, 1842) – Few strong ribs (~ 6–12); very spaced; distributed throughout the shell flank .................................. ......................................................................................... Cardiomya ornatissima (d’Orbigny, 1853)

5. Shell flank rounded ........................................................................................................................... 6 – Shell flank elliptical to trapezoidal ................................................................................................... 7

6. Rostrum long ............................................................................ Cardiomya perrostrata (Dall, 1881) – Rostrum very short .................................................................................................. Cardiomya sp. 1

7. Ribs (~ 30–50) equidistant, evenly distributed throughout the shell to the rostrum ........................... ...................................................................................................... Cardiomya striata (Jeffreys, 1876)

– Ribs (~ 18–35) more spaced in the posterior part of the shell flank.................................................... .................................................................................................... Cardiomya striolata (Locard, 1897)

8. Anterodorsal margin forming a shoulder .............................................. Cardiomya minerva sp. nov. – Anterodorsal margin descending ............................................................................. Cardiomya sp. 2