Key to species of Symmius 1. Pereopod 7 ambulatory................................................................................................................................2 - Pereopod 7 prehensil or rudimentary...........................................................................................................3 2. Body 3.1 times as long as wide; pereonites and pleonites without mid-dorsal projection; anterior angle of ocular lobe strongly projecting anteriorly ................................................................ S. caudatus Richardson - Body 2.5 times as long as wide; pereonites 1–7 and pleonite 2 each with mid-dorsal projection; anterior angle of ocular lobe not projecting ..................................................................... S. yamaguchiensis sp. nov. 3. Body 2.4 times as long as wide; pereopods very setose; pereopod 7 prehensil ........... S. planus Nunomura - Body 2.8 times as long as wide; pereopods scattered setae; pereopod 7 rudimentary .................................. .................................................................................................................................. S. philippinensis Poore

Published as part of Shimomura, Michitaka, 2008, A new species of Symmius Richardson (Crustacea: Isopoda: Chaetiliidae) from Japan, pp. 61-68 in Zootaxa 1719 on page 62, DOI: 10.5281/zenodo.181052