viii . PREFACE. 
already published with the newer work, and especially to render the volume 
more useful 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 limitations 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 much 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 added for the farther illustration 
of the genera Modioinorpha and Goniophora, are xxxv and xlii. The plates 
xciii-xcvi inclusive, were necessary for the illustration of new forms, and for 
