BEFORE turning to the decorous literary opening of the Arthurian cycle, one book demands attention, produced on British soil though not in the English tongue. Through it we can look down a long vista to the age of prehistoric origins, when already the fantasy of primitive Celts was playing around the person and the court of Arthur. The series of Welsh tales known as the Mabinogion, found in a fourteenthcentury manuscript known as the Red Book of Hergest, is a late reduction of early material. Five of the tales treat Arthurian subjects. Three of these, The Lady of the Fountain, Geraint and Peredur, are based on poems by Chrétien de Troies, though in Peredur, a strange version of the Perceval Grail-legend, barbaric traits are preserved unknown to the French poem, imparting a savage flavor to the aroma of the court, and the provenance has been questioned. The other two, Kilwch and Olwen and The Dream of Rhonaby, have no known models. They are pure detritus of myth, -- myth no longer crystallized, yet sparkling even in its confused disintegration with the genuine light of early imagination.