£ INTRODUCTION. 
Not the least of the pleasure we find in bringing our work to a conclusion, 
is the opportunity it affords us of acknowledging our obligations to the men 
of science and collectors of America, for their liberality and personal con- 
fidence shown to us, by placing in our hands — often for indefinite periods 
— original, unique, and priceless collections, without the use of which this 
work would have been impossible. It would be difficult to express in fitting 
terms of acknowledgment the full measure of our indebtedness to them, and 
we can only venture the hope that they may find in the work itself some 
small return for the valuable contributions they have made toward it. 
To Mr, Agassiz we owe a lasting debt of gratitude: first of all for his 
personal encouragement and valuable counsel, and next for the use of the 
magnificent collection of the Museum of Comparative Zoélogy. This has 
been placed at our disposal without restriction, not only for examination at 
Cambridge, but for removal to Burlington of all specimens we desired, with 
liberty to use them as if they were our own. Only those who are acquainted 
with the character and value of this unrivalled collection can appreciate our 
obligation for such a use of it. It contains the original collection of De 
Koninck, of the Belgian Carboniferous Crinoids, and the Schultze collection 
from the Devonian of the Eifel, —by far the finest collections that have 
ever been made of the rare Crinoids of those interesting localities. There 
are also the collections made by Hon. B. J. Hall, Prof. W. H. Barris, and 
the original collection of Wachsmuth, all from the Burlington limestone, 
which include the types of a large number of the species described by Hall, 
White, and Meek and Worthen. In addition to these is the fine collection 
made by C. B. Dyer, from the Hudson River group, of Cincinnati, con- 
taining many types of species described by Meek in the Ohio report, besides 
most excellent material from Waldron, Crawfordsville, and other celebrated 
localities of the West; also the Walcott collection from New York. 
A full account of the various collections made use of by us would 
exceed the limits of a preface, but we cannot refrain from making par- 
ticular mention of some of them: — 
The collections in the American Museum of Natural History at New 
York, containing many of the type specimens of the New York Paleonto- 
logical Reports, have been at all times accessible to us through the courtesy 
of Prof. R. P. Whitfield, who has been prompt to send us such specimens as 
we needed for illustration or comparison, and to give us any desired infor- 
mation obtainable from the extensive material under his charge. 
