Th is paper reports the results of investigations on the structure and texture of multilamellar ostracod carapaces and of comparative studies on the microstructure of the shell in Leperditiidae Jones and Healdiidae Harlton. The writer's inferences are that the multilamellar ostracod carapace has been derived from Conchostraca (Phyllopoda) with concentric growth lines. The consequent result is the separation of the group Eridoconchinae Henningsmoen into a distinct suborder - the Eridostraca. A new genus, Aberroconcha, has been established in this suborder and two species from the Middle Devonian of Poland a redescribed as new: Aberroconcha plicata and A. devonica. Moreover, one new species of Eridoconcha Ulrich & Bassler, equally of Devonian age, is described: Eridoconcha granulifera. A concept of the diphyletic origin of ostracods is postulated, based on the morphology and microstucture of their carapace.