PREFACE. xi 



was carried out so far as the lithographing of about thirty plates, when the 

 farther progress of the work was suspended, to be resumed only in the latter 

 part of 1888. 



In the meantime the duties of the author had separated him almost entirely 

 from this work, and owing to changes, over which he had no control, in 

 the organization and management of the State Museum, the collections which 

 he had planned to make for use in its preparation had not been made. The 

 progress in our current knowledge of the subject, and that recorded in the 

 publication of volumes and miscellaneous papers during more than twenty 

 years had been enormous, and the undertaking which had been deemed feasible 

 in 1867, seemed almost beyond attainment in 1888. The work was resumed 

 however, with no other collections immediately available for use, than those 

 upon which it had been commenced. In the original plan four plates were left 

 for the illustration of the Inarticulata ; the present volume furnishes ten 

 additional plates, and the illustrations of these forms may be regarded as fairly 

 complete, according to our present knowledge. 



The plates which were lithographed at the commencement of the work are 

 designated on the upper left-hand margin as " Volume IV, Part II." Those 

 lithographed since 1888 are designated as Volume VIII, and while the illustra- 

 tions, of the first named plates are not always arranged as would have been 

 done with later knowledge and more abundant material, it is hoped that the 

 intercalation of the new plates may not seriously interfere witli the proper 

 connection and continuity of the work, or with the facility of reference so 

 important to the student. Although the final numbering is XX, the actual 

 number of plates in the volume is forty-two. 



The printing of this volume had been completed to the end of the 

 Inarticulata, page 183, in March, 1890, when farther progress was suspended, 

 from causes over which the author had no control. The printing was resumed 

 in the autumn of 1890, and the book was in type to page 304 in February, 1891, 

 when its progress was again suspended to be resumed only in April, 1892. 

 This delay in publication, which has not in any way been due to the author, 

 requires an apology to the scientific public ; and those authors who may have 



