zii PREFACE. 



muiniiig portion, much of whicn has been executed in a better stylo 

 of art, has been done since 1855. 



The volume of plates has been much longer delayed than I could 

 have wished, and at the present time is not completed. This delay is 

 in great part owing to the fact of ascertaining, early in 1858, that many 

 drawings which I had entrusted to an experienced draughtsman to 

 make, were entirely worthless from their inaccuracy, some of them 

 even having been engraved, it became necessary to cancel the plates, 

 and to redraw and re-engrave the subjects. While this has delayed the 

 present volume, it has entailed upon myself no little personal expense 

 and trouble, besides seriously hindering the progress of the work for 

 the fourth volume. The later drawings will be found faithful and 

 accurate in all the details, and I hope may offer satisfaction to the 

 critical student in palaeontology. For the perfection of these drawings, 

 and for important aid in working out the details of structure in the 

 new genera of Brachiopoda, much credit is due to Mr. R. P. Whitfield. 

 I have been greatly indebted to Mr. John Patekson, for his supervision 

 of the work during its progress through the press. 



Considerable progress has been made in the work of the fourth vo- 

 lume ; and should there be no interruption in the present arrangement, 

 it will be published within a reasonable time. Owing to the cessation of 

 collections for several years, much has still to be done in obtaining 

 and preparing the materials for the work. Having, however, given 

 especial attention to the Brachiopoda of the higher rocks, a preliminary 

 notice of Vol. iv, so far as relates to this class of fossils, will appear in 

 1860. 



