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. Hayden, 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 Nebraska, 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 general assistant in the work for twenty years (1856 to 1876). 
After this date Mr. Charles D. Walcott, 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 Palieontology. He has also contributed to the State Museum re¬ 
ports a study on the Anatomy and Physiology of the Anodonta fluviaiilis. 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 
