2 INTRODUCTION. 
plates of the tegmen in the different groups. The first attempt in this 
direction was made by Wachsmuth in 1877, and the subject was taken up 
again in our Revision of 1879. Dr. P. Herbert Carpenter discussed the 
question more elaborately in 1884, when it became manifest that our views 
differed radically upon several important points, and especially as to the 
identification of the oral plates. The progress of our studies on this and 
other questions was published from time to time in the Revision, and in 
short papers. This was done for the double purpose of making known the 
results of our own studies, and of stimulating inquiry by others upon points 
that were still obscure. 
Whatever may be the merits or demerits of the Revision — and that the 
latter are many and serious none are better aware than we — it accom- 
plished one of its purposes. It induced research and provoked discussion 
upon ‘new lines and with an activity unprecedented in this field. The 
contributions to the knowledge of the subject, resulting directly from these 
controversies, have been of incalculable value to us, and none the less so 
because some of our own theories have been from time to time exploded. 
As the most important result, it has now become clear that the Crinoids 
were most intimately connected from the Silurian down to the present 
time, and that only the Camerata—a highly specialized type — became 
extinct at the close of the Carboniferous. It was not until this fact was 
realized that the way was opened to a better understanding of the whole 
Crinoid group, in which, as so often found in Nature, the simpler forms 
persisted, and led down to present types. Although it seems plain enough 
now, it was only by slow steps, and after long and patient research, that 
this result was reached. 
After a large amount of preliminary work had been done, we proceeded 
to prepare for publication in permanent form such part of it as we could 
reasonably hope to finish, and to that end we began the preparation of the 
illustrations in 1887. The work has grown upon our hands to such an 
extent that we found it necessary to limit it to the Camerata, the largest 
and most remarkable group among Palseozoic Crinoids. Thus limited, we 
could hope to give a reasonably full account of this group, and in connec- 
tion with discussion of the morphological and systematic relations of the 
other groups, to give some account of the Crinoids generally. 
The most of the drawings were made under our personal supervision in 
our Museum at Burlington; a few were made in Washington. Thirty-five 
