Genus Mediomastus Hartman, 1944

Type species: Mediomastus californiensis Hartman, 1944

Diagnosis (after Green 2002). Body small, thread-like. Proboscis papillated. Thorax with 9–11 chaetigers, usually 10. Chaetigers 1–4 with capillary chaetae in both rami. Remaining thoracic and abdominal chaetigers with hooded hooks. Sometimes, paddle-like chaetae may occur in the thorax and sometimes capillary chaetae may occur in notopodia of posterior abdominal segments. Pygidium terminal with mid-ventral caudal cirrus.