Existing identification keys (Lang 1948; Coull 1973; Willen 1996; Wells 2007) are either out of date or unsound due to misinterpretations or observational errors (see footnotes in Table 2). A new key is presented below which, in the absence of male information for most species, is inevitably based on female characters only. Single male specimens have so far been reported on only four occasions: Pseudomesochra gemina (Coull 1973; Willen 1996), P. perplexa (Bodin 1968), and Keraia tamara (Smirnov, 1946) (Smirnov 1946).
1. Antennule ♀ with conspicuous projection on posterior distal corner of segment 1; antennary exopod enlarged, exp-2 with at least one very long element at its tip; P1 exp-1 elongate, longer than exp-2 and -3 combined; outer elements of P1 exp-1 and -2 setiform; P2–P4 exp-1 with inner seta......................................... Keraia Willen & Dittmar, 2009 …..2
- Antennule ♀ without conspicuous projection on posterior distal corner of segment 1; antennary exopod not enlarged, with apical setae of distal segment not excessively long; P1 exp-1 not elongate, shorter than exp-2 and -3 combined; outer elements of P1 exp-1 and -2 spiniform; P2–P4 exp-1 without inner seta....................... Pseudomesochra T. Scott, 1902 …..5
2. P2 enp-2 with one or two inner setae...................................................................... 3
- P2 enp-2 with three inner setae................................................. K. longirostrata Apostolov, 2011
3. Antennule ♀ 5-segmented; P3–P4 exp-3 with two inner setae.................................................. 4
- Antennule ♀ 6-segmented; P3–P4 exp-3 with three inner setae............................ K. tamara (Smirnov, 1946)
4 Cephalothorax about 1.3 times as long as maximum width; apical segment of antennary exopod with shorter seta about half the size of longest one; P5 endopodal lobe ♀ with one lateral and three apical setae................................................................................................ K. longiseta (Vasconcelos, George & Santos, 2008)
- Cephalothorax about 1.1 times as long as maximum width; apical segment of antennary exopod with longest seta only slightly larger than shorter one; P5 endopodal lobe ♀ with all four setae arranged around apical margin................................................................................................ K. ricardae Willen & Dittmar, 2009
5 P2–P4 endopods 2-segmented........................................................................... 6
- P2–P4 endopods 3-segmented........................................................................... 8
6 P4 enp-2 with one inner seta; inner margin of caudal ramus with spinular patch near base........... P. affinis (Sars, 1920) *
- P4 enp-2 with two inner setae; inner margin of caudal ramus without spinular patch near base......................... 7
7 P2 exp-3 with inner seta; caudal ramus slightly expanded proximally and with concave outer margin; P5 endopodal lobe ♀ with three very long setae............................................................. P. longifurcata T. Scott, 1902
- P2 exp-3 without inner seta; caudal ramus not expanded at base and with straight inner and outer margins; P5 endopodal lobe ♀ with long middle seta flanked by two shorter ones, all of which markedly shorter than in preceding species............................................................................................ P. divaricata (Sars, 1906)
8 P1 exp-2 with inner seta................................................................................ 9
- P1 exp-2 without inner seta............................................................................ 16
9 Antennule ♀ 5-segmented; P1 enp-1 with inner seta.......................................... P. media (Sars, 1911)
- Antennule ♀ 6- or 7-segmented; P1 enp-1 without inner seta.................................................. 10
10 P2–P4 exp-3 with one inner seta; P2–P3 enp-3 with one inner seta; P4 enp-3 with three elements.... P. latifurca (Sars, 1911)
- P2–P4 exp-3 with two, three, three setae, respectively; P2–P3 enp-3 with two inner setae; P4 enp-3 with four elements.... 11
11 Antennule ♀ 6-segmented; P2–P4 exp-3 with two outer spines................................................ 12