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Clas Myrddin, or Merlin's Enclosure, is an early name for Great Britain given in the Third Series of Welsh Triads. It is implied that it is the oldest name, as opposed to "Albion", but the implication is not wholly credible.
References
Rhys: Hibbert Lectures, p. 168.
Stephen Thomas Knight, Merlin: Knowledge and Power Through the Ages, p. 6
