The Purbeck basal limestone beds in Dorset, southern England, have long been considered freshwater tufaceous deposits. Recognition of calcareous algae such as Ortonella, Girvanella, and Solenopora places them as marine algal deposits, and additional evidence from anhydrite, gypsum, and celestite, the occurrence of marine ostracods, and the sparsity of other fauna point to marine deposition but with higher than normal salinity. Possibly periodic influx of fresh or brackish water allowed temporary survival of a few mollusks. The identification of the ostracods previously with the freshwater genera Cypris and Candona has been an error. In the discussion it is pointed out that confirmation of marine deposition of middle and upper Purbeck beds has been furnished by the boron content of illite.