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 tho.se 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 Grammysidas, 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 clo.se 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 



