Key to the Japanese species of Trionymus (adapted and modified from DANZIG & GAVRILOV- ZIMIN 2015)

1 Head with one pair of cerarii; dorsal oral-collar tubular ducts of two different sizes. ......... T. okiensis sp. nov.

‒ Head without cerarii; oral-collar tubular ducts absent from dorsum. ............... T. kobotokensis Kanda, 1959