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Llandeilo and Caradoc beyrichiocope Ostracoda are described from England and Wales. They are referred to 3 suborders (Palaeocopa, Binodicopa, Eridostraca), 8 families (Eurychilinidae. Ctenonotellidae. Tetradellidae (with 1 new subfamily); Bolliidae, Aechminidae, Circulinidae and Spinigeritidae; Eridoconchidae). 38 genera (7 new and 2 new subgenera) and 52 named species (35 new). Thirteen other forms are referred mainly to the Palaeocopa under open nomenclature, or are simply recorded. Sampling was concentrated on the sandstone/limestone sequences of Llandeilo and Caradoc age of South Wales. Superbly preserved silicified faunas were extracted using acid techniques. Casting methods extended sampling into the predominantly sandy facies of Caradoc age of the Welsh Bc.;erland, North Wales and northern England. Largely neglected,these ostracode mould faunas have provided an excellent source of material. Scanning electron microscopy has been extensively employed to illustrate the ostracodes recovered. The biostratigraphy of Llandeilo and Caradoc beyrichiocopes from England and Wales is documented. They are useful in local stratigraphic correlation and may prove helpful in defining the base of the Caradoc Series in South Wales. Their potential for inter-regional correlation is limited. British Ordovician ostracodes support other biogeographic evidence that during Llandeilo to Ashgill times, southern Britain moved closer to the Baltic and North American plates, as indicated by a breakdown in the provinciality of the British faunas.