In the years 1899, 1900, and 1901 Mr. George Bonsor, a British subject residing in Spain, visited the Scilly Isles, the archipelago of tiny granite islands 28 miles south-west of Land's End, for archaeological research. Mr. Bonsor was long a profound student of the archaeology of his adopted country and was interested in the history of the ancient tin trade. It was in this connexion that he visited the Scillies, for he wished to test the hypothesis that they were the Cassiterides, the fabled tin islands of the Atlantic. With regard to the tin trade, his researches proved entirely negative, but in the course of his visits he excavated and planned some of the numerous megalithic tombs that abound in the islands. Mr. Bonsor, however, never published an account of his work in Scilly, but kept the notes and finds at his castle at Mairena del Alcor near Seville for many years. There he was visited in 1926 by Mr. T. D. Kendrick of the Department of British and Mediaeval Antiquities at the British Museum, and through him sent a part of the finds to the Museum. At the same time the writer had become interested in the archaeology of the islands, and with the help of Mr. Kendrick was able to obtain more of the finds for the British Museum as well as plans of the more important tombs and some notes with regard to their excavation. The most important plans and an abstract of the notes the writer has included in his Archaeology of Cornwall, but space prevented the use of all of Mr. Bonsor's material. Mr. Reginald Smith, Keeper of British and Mediaeval Antiquities at the British Museum, had for a long time urged Mr. Bonsor to communicate the results of his excavations to the Society of Antiquaries, but the latter died without having done so. Subsequently Mr. Smith obtained Mr. Bonsor's plans and the full account of the excavation of the most important tomb, and these he has kindly permitted the writer to publish.