Genus Mediomastus Hartman, 1944

Type species: Mediomastus californiensis Hartman, 1944, by original designation.

Diagnosis (after Warren et al. 1994). Prostomium conical, palpode present; eyespots present or absent, when present in multiple spots. Peristomium clearly distinct from prostomium; achaetous segment absent. First chaetiger biramous. Nine to eleven thoracic chaetigers but usually ten. Chaetigers 1–4 with only capillaries and remaining thoracic chaetigers with long-shafted hooded hooks. Sometimes paddle-like chaetae may be present in thorax and sometimes capillary chaetae may be present in notopodia of posterior segments. Thorax not clearly demarcated from abdomen. Abdominal segments with only short-shafted hooded hooks. Branchiae absent. Genital pores in three pairs from chaetigers 7/8 or 8/9 but difficult to observe. Lateral organs present on thorax and inconspicuous in abdomen. Pygidium with ventral caudal cirrus.