The indulgent student will please notice the following for the new edition _North American Slime Moulds_-- On p. 63, No. 17, read _Physarum megalosporum_ Macbr. Last line should read 1917 Physarum _melanospermum_ Sturgis, _Mycologia_, Vol. IX, p. 323. On p. 67, last line but one, at the end, read, p. 323. On p. 67, insert just before No. 23, Vicinity of Philadelphia,--_Bilgram_. On p. 327, Plate XIII, lacks numbers. These may readily be supplied by consulting descriptive text. On p. 344, in explanation figure 2, last word read hour. On p. 346, for name of species read _Fuligo rufa_ Pers., p. 28. PREFACE TO THE FIRST EDITION[1] The present work has grown out of a monograph entitled _Myxomycetes of Eastern Iowa_, published by the present author about eight years ago. The original work was intended chiefly for the use of the author's own pupils; but interest in the subject proved much wider than had been supposed, and a rather large edition of that little work was speedily exhausted. At that time literature on the subject in question--literature accessible to English readers--was scant indeed. Cooke's translation of Rostafinski, in so far as concerned the species of Great Britain, was practically all there was to be consulted in English. In 1892 appeared in London Massee's _Monograph of the Myxogastres_, and two years later in the same world's centre the trustees of the British Museum brought out Lister's _Mycetozoa_. Although these two English works both claim revision of the entire group under discussion, the latter paying special attention to American forms, nevertheless there still seems place for a less pretentious volume which for American students shall present succinct descriptions of North American species only. The material basis of the present work consists of collections now in the herbarium of the State University of Iowa. In accumulating the material the author has had the generous assistance of botanists in all parts of the country, from Alaska to Panama, and the geographical distribution is in most cases authenticated by specimens from the localities named. The descriptions, in case of species represented in Europe, are based upon those of European authors; for forms first described in this country, the original descriptions have been consulted. A bibliography follows this preface.