INTRODUCTION. 3 
of the plates were drawn by Dr. Charles R. Keyes, the present State geolo- 
gist of Missouri; thirty-three by Mr. A. M. Westergren, so well known for 
his drawings for Lovén’s great work on the Echinoids; the remaining 
twelve by Mr. John R. Ridgway, artist for the United States Geological 
Survey. The execution of the plates occupied about six years, and we 
avail ourselves of this opportunity to express our thanks to all of these 
gentlemen for the fidelity and earnestness with which they performed 
their work. 
When the work began to assume a definite shape, Mr. A. Agassiz, on 
being made acquainted with the extent to which it had progressed, kindly 
offered to undertake its publication as a part of the Memoirs of the Museum 
of Comparative Zoélogy at Harvard College. No words of thanks would 
at all express our sense of the obligation under which this has laid us, not 
merely for the facility of publication through so desirable a medium, but 
for the mark of appreciation which this offer implies. If the work shall be 
found sufficiently useful to science to merit, even in a small degree, the 
indorsement thus given, we shall deem it the best return we can make. 
During the studies that led up to this Monograph, we enjoyed the privi- 
lege of continued communication with our lamented friend, P. Herbert 
Carpenter, up to the time of his decease. We had some energetic con- 
troversies in print, and a far greater number in private correspondence that 
never saw the light. To his incisive and suggestive mind is due the over- 
throw of more than one promising but untenable theory; and we take a 
melancholy pleasure in recording here our appreciation of his high attain- 
ments, and our sense of the great loss which Science has suffered through 
his untimely death. 
It has been our purpose to give descriptions of all American species of 
the Camerata known up to this date, and those that could be recognized 
have been described anew, with the aids derived from the material brought 
to light since the original descriptions were made. Many of the species 
were defined from very imperfect specimens, and often without illustrations. 
In the latter cases we have, when practicable, figured the type specimen, 
and when necessary and possible have given figures of additional specimens. 
During the preparation of the work we have had access to most of the 
type specimens in the United States and Canada, which were placed in our 
hands for comparison, study, and illustration. A few only of Prof. Hall’s 
types in the New York State Cabinet of Natural History at Albany, and 
some of §S. A. Miller's later species, we were unable to procure. 
