"The Hannatoma fauna is an assemblage of marine and brackish-water mollusks present in Tertiary strata at several widespread points in northwestern South America. The distinctiveness and wide geographic range of the fauna combine to make it important in stratigraphic correlation, but its value in this respect has been impaired by difference of opinion as to whether its age is middle Oligocene, as originally claimed, or upper Eocene. The seven papers in this symposium comprise the response to a circular requesting current opinions as to the age of the Hannatoma fauna. Different aspects of the age-relationships of associated mollusks and Foraminifera in Venezuela, Colombia, Ecuador and Peru are discussed. The authors conclude that the Hannatoma fauna occurs mainly in upper Eocene but locally in Oligocene beds."