Field examination of the Carboniferous limestone at Nant y Gamar, southeast of Llandudno on the coast of north Wales, indicates that fenestrate bryozoa are the major organic components of a shoal or knoll-like structure in limestone of S 2 age. The limestone is a bryozoan calcirudite consisting of large, intact or occasionally broken fenestrate bryozoa in a matrix of dark brown microcrystalline calcite containing fine grained crinoid and brachiopod fragments, Foraminifera and calcispheres (biomicrudite, Folk, 1959). The bryozoan limestone is flanked by crinoidal calcirudites, slightly quartzose bioclastic calcarenites with matrices of microcrystalline calcite (respectively biomicrudite and biomicrite, Folk, 1959), and intraclastic calcirudites with calcite cement (intrasparudite, Folk, 1959). In view of the structure and lateral variation of facies, it is tentatively proposed that the bryozoan rock unit represents a shoal or possibly a fore-reef deposit of a small reef-knoll. The bryozoa do not appear to be in a growth position, or to form a rigid framework, but seem to represent a loose mesh of zoaria derived by transportation or scavenging action. It is assumed that they were continually derived from a nearby reef or broken in situ, and entrapped sediment transported by currents in shallow water, thus forming a prominence. The prominence grew by accumulation, and resulted in a shoal or knoll-like structure.