Key to all known species of Pseudonymus Diesing, 1857
1. Two cephalic cuticular annule........................................................................... 2
- More than two cephalic cuticular annule................................................................... 5
2(1). Female tail conical................................................................................... 3
- Female tail filiform................................................................ P. leptocercus Todd, 1944
3(2). Eggs 78–88 µm in length.......................................................... P. brachycercus Todd, 1944
- Eggs less than 78 µm in length........................................................................... 4
4(3). Three pairs of caudal papillae.............................................. P. hydrophili (Galeb,1878) Basir, 1956
- Six pairs of caudal papillae........................................................ P. basiri Shah & Rizvi, 2004
5(1). Six cephalic cuticular annule.......................................................... P. klossi Farooqui, 1967
- More than six cephalic cuticular annule................................................................... 6
6 (5). Female tail filiform.................................................................................... 7
- Female tail conical.................................................................................... 8
7(6). Nine cephalic cuticular annules.......................................... P. spirotheca (Gyory, 1856) Diesing, 1857
- 30 cephalic cuticular annules.................................................... P. multiannulata Fotedar, 1964
8(6). Less than 60 cephalic cuticular annules................................................................... 9
- More than 60 cephalic cuticular annules............................................ P. reuhmi Gupta & Kaur, 1978
9(8). Oesophagus 0.45 mm in length............................................ P. islamabadi (Basir, 1941) Basir, 1956
- Oesophagus less than 0.45 mm in length................................................................. 10
10(9). Three pairs of caudal papillae.......................................................... P. vazi Travassos, 1954
- Five pairs of caudal papillae............................................... P. toddi (Travassos, 1954) Kloss, 1959