Sars, 1835, the type species of the genus, was originally described from the south-west coast of Norway. Over the past 170 plus years this species has been reported from around the world often without a detailed description; in some cases, schematic illustrations of the anterior end with the distinctive branchiae were given. Identifications were likely based mainly on the branchiae consisting of 4–5 lobes but recent work has shown that this character alone has led to misidentifications and confusion of a number of morphologically similar species. Michael Sars (1835) did not designate type specimens for this species. In order to provide an accurate definition of the genus it is necessary to redescribe and characterize the type species including the designation of a neotype. This will provide a baseline against which the potentially undescribed sibling species in the area can be compared and fully described. A large collection of specimens identified as T. stroemii
from locations along the coasts of Norway was studied in the Natural History Museum, University of Oslo. This has prompted us to locate material collected by M. Sars (1835) from the vicinity of the type locality and to designate a neotype and provide a comprehensive description of the type species of the genus.