INTRODUCTION. 



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 Loven's great work on the Echinoids ; the remainincr 

 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 Zoology 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 fight 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. 



