This paper presents a critical assessment of killingley´s approach to the determination of onth of collection of marine molluscs by prehistoric people. The basis of the method is careful oxygen isotope measurements made in successive growth increments in the shells. We have analyzed specimens of Monodonta and Patella collected live on the coast of northern spam, in conjunction with seasonality studies on molluscs from prehistoric sites in the meighborbood. These studies confirm the necessity of making careful analyses of each species under consideration. Given the significance both of interspecies differences and of climatic variability on timescales from a year upwards (particulary important in killingley´s area), we conclude that his apparent accuracy of a month is ilusory.