LYONNESSE, Lyonesse, Leonnoys or Leonais, a legendary country off the south coast of Cornwall, England. Lyonnesse is the scene of many incidents in the Arthurian romances, and especially in the romances of Tristram and Iseult. It also plays an important part in purely Cornish tradition and folklore. Early English chronicles, such as the Chronicon e chronicis of Florence of Worcester, who died in 1118, described minutely and without a suggestion of disbelief the flourishing state of Lyonnesse, and its sudden disappearance beneath the sea. The legend may be a greatly exaggerated version of some actual subsidence of inhabited land. There is also a very ancient local tradition, apparently independent of the story of Lyonnesse, that the Scilly Islands formed part of the Cornish mainland within historical times.
See Florentii Wigorniensis monachi Chronicon ex chronicis, &c., ed. B. Thorpe (London, 1848–1849).