Type species: Notomastus latericeus Sars, 1851 by monotypy.
Diagnosis (after Green, 2002). Thorax with 12 segments including an achaetous peristomium and 11 chaetigers with capillary chaetae. Last thoracic chaetiger may or may not have capillary chaetae in neuropodia. Remaining chaetigers with hooded hooks. Hooded hooks with one or more rows of teeth above main fang; more than two teeth in basal row. Branchiae may be present or absent.
Remarks. The definition of Notomastus has changed to include other genera with eleven thoracic chaetigers and hooded hooks in the neuropodia of the last thoracic chaetigers (Ewing 1982; Blake 2000). Green (2002) separates Notomastus species from the Capitellethus-Leiochrides-Leiocapitella complex based on the number of spinules/hooks in the basal row of the hooks. We follow this perspective as a revision of species and genera has not been done using SEM to clarify their affinities.