PREFACE. 
IX 
the correction or farther elucidation of forms already lithographed in the pre¬ 
ceding plates. 
Since that period, the material in the collections has received some accessions 
and a very careful examination. The genera and species have been critically 
studied and extensive revision has been made in the limitations of both. A 
careful examination of those species supposed to have a vertical range from the 
Chemung Group to the Waverly Group or the Burlington Yellow Sandstone, 
has shown that they are allied forms, but specifically distinct. 
In many cases the differences are not strongly marked, but are constant and 
uniform, and are such changes as might very well come with the lapse of time, 
the slight change in the nature of the sediments, and the geographical relations 
of the localities. The genera prevalent in the Waverly Group are identical 
with those of the Chemung, and in all the collections which have come under 
observation, there are but four genera of Lamellibranchiata of the former group 
which are not also common to the Hamilton or Chemung. 
The slow and gentle gradation of generic forms is well illustrated in the 
succession of strata from the Hamilton to the Waverly, and in no group so 
distinctly as in the Grammysidae, as will be shown under the descriptions 
of genera in the following pages. 
The nature of the sediments and the characters of the fauna from the 
beginning of the Hamilton Group to the close of the Waverly give evidence 
of gradually encroaching dry land on the eastward, or shallow sea, and off¬ 
shore deposits. This condition culminated in the eastern area by the great 
accumulation of the coarse sediments of the Catskill Group. 
The conditions of preservation of the Lamellibranchiate shells rarely admit 
of the study of their interior structure, and the external form and sculpturing 
must be relied upon to a great extent in the determination of generic relations. 
It has happened, however, in a considerable number of forms, that we have 
been able to obtain illustrations of the hinge characters, leaving no doubt of 
the relations of the fossil. In many cases the surface sculpturing is so distinct 
and peculiar that we may group the forms together without hesitation. 
In the progress of the work to its final revision, it has been found necessary 
B 
