XII 
PREFACE. 
maining portion, much of which has been executed in a better style 
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 Paterson, 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. 
