Conchostracans associated with bivalves, gastropods, ostracodes, fish scales and plant remains (mainly glossopterids and sphenophytes) from 67 outcrops of the Rio do Rasto Formation (Passa Dois Group, Parana Basin, Upper Permian) in Parana and north of Santa Catarina States occur as molds of frequently flattened and not dismembered carapaces (or sometimes isolated valves) in ritmic or massive red, gray and yellowish mudstones, siltstones and fine sandstones. Asmussia regularis and Monoleaia unicostata, previously reported from only a few localities of this formation, in addition to one new genus and species - Paranaleaia supina, are the predominant species and thus have a much broader geographical distribution than previously thought. Other identified species include: Asmussia sp., Cyzicus (Euestheria) sp., C. (Lioestheria) sp., Cyzicus sp., Gabonestheria sp., Palaeolimnadiopsis subalata, cf. Palaeolimnadiopsis sp. and Palaeolimnadia (Palaeolimnadia) sp.. Probable egg capsules were found in association with a carapace of Monoleaia unicostata. The postero-dorsal margin of this species is sometimes preserved as a narrow area with fine longitudinal grooves, possibly related to the articulation of the valves. Two very distinct types of carapace sculpture present in different specimens of this same species may support a separation in two new subspecies: M. unicostata timbcensis and M. unicostata micropolygnata
