The straight-hinged Paleozoic ostracods are assigned to the new suborder Paleocopa (superfamilies Beyrichiacea and Leperditiacea) and the suborder Podocopa (family Quasillitidae) . New taxonomic units are: family Sigmoopsiidae, subfamilies Sigmoopsiinae, Glossopsiinae, Beyrichiopsiinae, Conchoprimitiinae, Eridoconchinae, and the genera Sigmoopsis, Sigmobolbina, Carinobolbina, Bolbina, and Hesslandella. In the classification of the Beyrichiacea more attention is paid to type of dimorphism, submarginal structure (velate and carinal structures) , and unusual features than to the degree of sulcation and lobation. Probable phylogenetic lines from trisulcate to unisulcate fonns suggest that trisulcate fonns need not be more advanced than uniculcate fonns. Moult retention is described in the Conchoprimitiinae, and its bearing on the study of ontogeny and detennination of orientation is discussed. The proposed classification of straight-backed Paleozoic ostracods is presented in a list (p. 266) . A range chart (p. 276) shows the stratigraphical distribution of the families and subfamilies. The Healdiidae and Thlipsuridae are believed to be related to straight-hinged Podocopa (Quasillitidae) . The relationships of the Paleocopa to other ostracod suborders are still uncertain.