Key to species of Megaesthesius

1. Outer margins of ischium and merus of third maxilliped unarmed (Fig. 34 B). Ventral margin of chela unarmed, pollex rela- tively slender; cutting margins of fingers of major male chela unarmed (Fig. 48 E) [Japan; East China Sea]...... M. yokoyai

- Outer margins of ischium and merus of third maxilliped lined with small teeth or spines (e.g., Fig. 34 A). Ventral margin of chela dentiform, pollex relatively stout; cutting margins of fingers of major male chela with small teeth (e.g., Fig. 48 C)....2

2. Lateral margins of carapace with numerous prominent sharp teeth and spines (Fig. 19 A, B) [Singapore; Vietnam; South China Sea; questionably from Andaman Sea]............................................................ M. sagedae

- Lateral margins of carapace with lobiform teeth, the margins can be armed with spinules (e.g., Figs. 19 M, 20A, B)........3

3. Fingers of chela prominently longer than palm (Fig. 48 D). Ambulatory legs, especially meri, conspicuously long, relatively slender (Fig. 20 A, B) [Philippines].......................................................... M. migmus n. sp.

- Chela unknown. Ambulatory legs, especially meri, relatively shorter (Fig. 19 Q, R) [Western Australia]....... M. westralia