viii PREFACE. 



already published with the newer work, and especially to render the volume 

 more usel'ul for reference and comparison, since much confusion has existed 

 regarding generic relations. 



The original plan, as thus proposed, cannot now be carried out, owing to the 

 many delays which have come from extraneous sources, and the serious inter- 

 ference of other duties not contemplated at the time of its inception ; and more 

 especially the interruption of the progress of the work for many years and its 

 revival under lianitations precluding the employment of the time and means 

 necessary for the acquisition of material required for the proper illustration 

 of the subject. Moreover, the types of many of the genera are widely distant, 

 and could only be made available by visiting the museums or collections in 

 which they are preserved. 



The author, therefore, finds himself obliged to publish the work in its incom- 

 plete condition ; trusting that what has been accomplished may prove of 

 assistance to those who follow him, and that it may stimulate some one, with 

 better means for undertaking the work, to complete a monograph of the 

 Palaeozoic Lamellibranchiata. 



The comparatively few species from the Waverly Group, given in this volume, 

 with the more numerous forms elsewhere published from that horizon, and from 

 the Carboniferous rocks above, give some idea of what might be obtained by 

 systematic working in the upper palaeozoic formations. 



It was at one time the ambition of the author to accomplish this work ; and 

 it has been partly with this object in view that he has taken nmch pains to 

 establish the line of demarkation between the Chemung and the succeeding 

 Waverly Group ; which limitation was not well understood when the present 

 work was begun more than fifteen years since. 



The remarks in the third to the seventh paragraph of the prefaces of the 

 preceding volume are equally applicable to the present one. With the exception 

 of a few plates, the work of lithography had been completed in 1876. While 

 the work has been under revision, the plates jidded for the farther illustration 

 of the genera Modiomorpha and Goniophora, are xxxv and xlii. The plates 

 xciii-xcvi inclusive, were necessary for the illustration of new forms, and for 



