A new genus and species of nereidid (
Lycastilla cavernicola) is described from a freshwater pond within a cave found in Mexico. Its affinities with the other five genera of the subfamily are discussed with comments on the genus
Namanereis. The new genus has more affinities with
Lycastoides Johnson than with other genera in the subfamily despite the difference in the structure and number of tentacular cirri, and is presumably a relict species. The habitat is described and some remarks on the biology of the species are given.
