PREFACE. 



XV 



During the fifty-one years which have elapsed since the commencement of 

 this work, I have had many assistants who directly or indirectly have aided 

 in, or have contributed to its progress. Among the earliest of these was Mr. 

 Fielding B. Meek (afterwards Palaeontologist to the United States Geological 

 Survey of the Territories), whose services were largely given to the drawings 

 for the plates of Volume III, which were lithographed by Mr. Frederick J. 

 SwiNTON, the latter continuing his connection with the work till 1872, enriching 

 the volumes by his excellent artistic work. During the early part of the same 

 period, Mr. Ferdinand V. Havden, who subsequently became Director of the 

 U. S. Geological Survey, was my assistant, and, together with Mr. Meek, made 

 a survey of the Mauvaises Terres of Nebrasl^a, at my personal expense. Dr. 

 Charles A. White, now of the National Museum at Washington, who had been 

 my assistant in the Iowa Survey, was, for one year, engaged in the service of 

 the Palaeontology of New York, in making field collections and obtaining 

 geological data. Mr. Robert P. Whitfield, now Curator of Geology in the 

 American Museum of Natural History, was associated with me as preparateur, 

 draughtsman and genei*al assistant in the work for twenty years (1856 to 1876). 

 After this date Mr. Charles 1). AValcott, now Director of the U. S. Geological 

 Survey, became my assistant for two years. In the final revision and publication 

 of the four volumes, which constitute Volume V, I had the assistance of Mr. 

 Charles E. Beecher, now Professor in Yale University, from the commence- 

 ment of the Cephalopoda to the completion of the Lamellibranchiata, from 

 1878 to 1885. Mr. George B. Simpson, who has served the work for many 

 years as draughtsman, has made himself very familiar with the Bryozoa and 

 Corals of our geological formations, and has given very essential aid in the 

 preparation and publication of Volume VI, as well as in other work connected 

 with the Palaeontology. He has also contributed to tlie State Museum re- 

 ports a study on the Anatomy and Physiology of the Anodonta Jluviaiilis. In 

 the capacity of my private assistant, the services of Mr. Charles Schuchert, 

 now of the U. S. National Museum, were given to the forwarding of Volume 

 VIII, as already stated in the Preface to Part I. Professor J. M. Clarke, who 

 came into the work in 1886, has given essential aid in the preparation of 



