two benthic mollusc assemblages of the continental shelf on both sides of the atlantic ocean, a tropical one in rio de janeiro, brazil, and another, temperate, in galicia, spain were investigated, with a view to finding common environmental descriptors which would explain, on a macro-scale, why these assemblages are there. both of the assemblages concerned show approximately the same species richness, about 150 taxa each. the molluscan fauna of both regions live on sandy sediments. the galician assemblages are at about 2-12 m depth, while those in rio de janeiro are at about 10-40 m depth. malacological assemblages were defined through cluster analysis and multiple discriminant analysis of the environmental data showed that each assemblage has its own environmental space. these assemblages have no species in common, but show the same phenological characters associated with each sedimentological facies. the same set of environmental variables (median sediment grain size, skewness, kurtosis, sorting, fine and medium sand fractions and depth) were selected as controlling these assemblages, suggesting that they play their role as general environmental descriptors.