SILURIAN AND DEVONIAN MOLLUSCAN BIVALVES FROM PRECORDILLERA REGION, WESTERN ARGENTINA

Silurian and Devonian bivalves recovered from the Los Espejos (Siluria n to Lower Devonian) and Talacasto (Devonian) Formations of the Northe rn Precordillera Mountain belt are described and illustrated. The faun a comprises some new nuculoids Deceptrix (Devonodeceptrix) jachalensis n. subgen. and n. sp., Nuculites argentinum n. sp., and Anthracoleda (Pseudoleda) minuta n. subgen. and n. sp., as well as the new pterioid Actinopteria modesta n. sp. Additionally, many poorly known South Ame rican bivalves are redescribed and figured. Silurian-Devonian pelecypo d faunas from some South American localities show differences both in diversity and in faunal composition. Causes for these differences are briefly discussed. Controls other than latitude are suggested in order to explain biogeographical affinities of the studied fauna.