Pisoids from the Lower Carboniferous Llanelly Formation of South Wales contain both algal and fascicular-optic calcite laminae. The latter are here interpreted as a replacement product of aragonite and display characteristic calcitization textures. The original aragonite laminae formed in a hypersaline setting and are compared with Recent coniatoid deposits. The pisoids also show evidence of having grown periodically in a near normal marine setting as well as showing evidence of early solution probably by meteoric waters. The pisoids grew in a schizohaline lagoon formed as the initial phase of a marine incursion, during a period of fluctuating peritidal and arid floodplain deposition. The schizohalinity resulted from seasonal effects in a climate with a marked dry season.