Discussion................................................................................................................................................................ 433 Conclusions...................

Thirteen species of fossil molluscs are reported from the Solimões Formation of western Brazilian Ama-zonia. Based on mammalian chronology of the Solimões Formation and radiometric ages reported from coeval deposits in adjacent Peru, the age of the fauna is established as Late Miocene. The fauna includes fi ve prosobranch gastropod species, seven pearly freshwater mussel species and one sphaeriid bivalve species. The supposed presence of Pachydon (Corbulidae: Bivalvia) in these deposits is rejected; Pachy-don acreanum, whose status has long been uncertain, is transferred to the unionoid genus Callonaia. The Solimões mollusc fauna is entirely composed of obligate freshwater taxa, resembling species-poor mod-ern Amazonian fl uvial faunas. The presence of the fauna in outcrops covering large parts of western Amazonia indicates that by that time the preceding Pebas fauna (dominated by corbulid bivalves and cochliopid snails) must have been extinguished