Lyonnesse is the scene of many incidents in the Arthurian romances, and especially in the romances of Tristram and Iseult.
LYONNESSE, LYONESSE, LEONNOYS or Leonais, a legendary country off the south coast of Cornwall, England.
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.
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.
Somewhat similar legends are those of the island of Brazil, of Lyonnesse, the sunken land off the Cornish coast, of the lost Breton city of Is, and of Mayda or Asmaide - the French Isle Verte and Portuguese Ilha Verde or "Green Island" - which appears in many folk-tales from Gibraltar to the Hebrides, and until 1853 was marked on English charts as a rock in 44° 48' N.