The subfamily Koneprusiinae is revised as part of a larger study of the Odontopleuridae, and Laethoprusia gen. nov. is described. The type species L. salax sp. nov. is from the Wenlock of Gotland, Sweden, and the genus is known also from the Wenlock of the Montagne Noire in south France, and from the Lower Devonian of Australia. The type species is most unusual among trilobites in possessing large fenestrae perforating the exoskeleton on segmental boundaries from the cephalic posterior margin to within the pygidium. The fenestrae were probably covered with soft tissues. Their function is enigmatic. Laethoprusia gen. nov. is the sister taxon to Isoprusia and/or Koneprusia. These genera are revised, and a review of all species assigned to them is given. A cladistic analysis of the best known taxa of Laethoprusia, Isoprusia, and Koneprusia is presented. Laethoprusia is the most derived genus of the subfamily, although stratigraphically it is the earliest.
