I must express my appreciation of the courteous attentions of the accomplished Librarian of Congress, the Hon. A. R. Spofford, who together with his assistants did everything possible to facilitate my investigations. To the uniform and friendly interest which Mr. Spofford has manifested in my work, its successful completion is largely due. The substantial assistance which I received from the lamented Professor Joseph Henry—the record of whose kindly offices to his fellowmen can never be written—was invaluable to me. Besides placing the latest material at my disposal, he generously furnished most of the engravings in this work relating to the Mound-builders. Dr. Charles Rau, also of the _Smithsonian Institution_, has placed me under obligations for valued services. To Professor F. V. Hayden and to the painstaking offices of Mr. James Stevenson of the _U. S. Geological and Geographical Survey of the Territories_, I am indebted for the engravings as well as the sources of information relating to the Cliff-dwellers. The Hon. J. R. Bartlett, of Providence, R. I., with equal generosity has conferred like favors. Prof. F. W. Putnam, of the Peabody Museum of American Archæology and Ethnology at Cambridge, Mass., and his courteous assistants, Mr. Carr and Miss Smith, have provided me with valuable engravings and reports. Robert Clarke, Esq., and Mr. E. Gest, of Cincinnati, have also sent me engravings, and the former in particular has conferred frequent favors. Professor Ph. Valentini, of Albion, N. Y., with rare liberality, contributed interesting material relating to the Nahua Calendar. To Mr. Stephen Salisbury, Jr., of Worcester, Mass., Dr. R. J. Farquharson, of the Davenport Academy of Sciences, Rev. S. D. Peet, editor of the _American Antiquarian_, Cleveland, O., and to A. J. Conant, Esq., of St. Louis, Mo., I am indebted for the interest they have manifested, and for the material which they have brought to my attention. Señor Orozco y Berra, of the City of Mexico, the distinguished author of the _Geografía de las lenguas Mexicanas_, has from time to time freely made important suggestions concerning some of the problems under consideration. To my friend the Rev. John W. Butler, of the City of Mexico, whose intelligent efforts in my behalf have been unremitting, I have special reason to be thankful. To all these generous friends I must be permitted here to express my deep sense of gratitude for their favors.