This paper records a few geographic and archeologic observations related with several "sambaquís" found in the lagoon region of Cananéia, in the south coast of the State of S. Paulo. The major part of such shell-mounds were found to be lying directly upon beach or dune sand, their location being, generally, at the margin of a "marigot" (salt-water river). From the observations made at four different "sambaquís" the authors have concluded that these mounds may be considered a most valuable material for the investigation of the region's near past history. It is believed that at the time the "Man-of-the-sambaquí" was living, the planimetric configuration of the Isle of Cananéia and other low parts of the region were considerably different than at the present time. The authors point out that all the former controversies between the "naturalist" and "artificialist" currents have been definitively settled by the work of Guerra (1951, p. 3-18) in which the differences existing between the "sambaquís" and the "wave-built terraces" are demonstrated by this geographer. The lagoon region of Cananéia with its "restingas", canals, etc., is thoroughly analysed and the probable configuration of the lagoon system and marine canals during the prehistoric period is examined. After a few speculative considerations about the prehistory of the region and after studying the stratification of the "sambaquís" layers together with the heterogenous materials which are included in them the authors present their hypothesis that the building of these mounds was probably connected with magic ritual practices.