material available in print in libraries of this country and arranged topically, but I did not publish it, as it seemed advisable to supplement it by study abroad and of the manuscript material, and to adopt an arrangement by authors. The result is Books IV and V of the present work. My examination of manuscripts has been done especially at the British Museum, whose rich collections, perhaps because somewhat inaccessibly catalogued, have been less used by students of medieval learning than such libraries as the Bodleian and Bibliothèque Nationale. I have worked also, however, at both Oxford and Paris, at Munich, Florence, Bologna, and elsewhere; but it has of course been impossible to examine all the thousands of manuscripts bearing upon the subject, and the war prevented me from visiting some libraries, such as the important medieval collection of Amplonius at Erfurt. However, a fairly wide survey of the catalogues of collections of manuscripts has convinced me that I have read a representative selection. Such classified lists of medieval manuscripts as Mrs. Dorothea Singer has undertaken for the British Isles should greatly facilitate the future labors of investigators in this field. Although working in a rather new field, I have been aided by editions of medieval writers produced by modern scholarship, and by various series, books, and articles tending, at least, in the same direction as mine. Some such publications have appeared or come to my notice too late for use or even for mention in the text: for instance, another edition of the _De medicamentis_ of Marcellus Empiricus by M. Niedermann; the printing of the _Twelve Experiments with Snakeskin_ of John Paulinus by J. W. S. Johnsson in _Bull. d. l. société franç. d’hist. d. l. méd._, XII, 257-67; the detailed studies of Sante Ferrari on Peter of Abano; and A. Franz, _Die kirchlichen Benediktionen im Mittelalter_, 1909, 2 vols. The breeding place of the eel (to which I allude at I, 491) is now, as a result of recent investigation by Dr. J. Schmidt, placed “about 2500 miles from the mouth of the English Channel and 500 miles north-east of the Leeward Islands” (_Discovery_, Oct., 1922, p. 256) instead of in the Mediterranean.