XX 
INTRODUCTION. 
In some localities we have the most positive evidence that the sediments 
have been deposited in circumscribed areas, as in the case of the Panama con¬ 
glomerate and the Salamanca sandstones, and in these examples we have a 
peculiar and restricted fauna. 
In its far western extension the Chemung group has become a calcareous 
formation, bearing many of the characteristic forms of Brachiopoda and but few 
of the Lamellibranchiata so characteristic of the group in the east. 
The Waverly sandstone and its equivalent or representative formations in 
the west, have not yet been studied with that degree of care and comparison, 
over wide areas, which are sufficient to warrant generalizations regarding the 
distribution of the Lamellibranchiate fauna. The species described from 
widely separated localities are a sufficient evidence that the conditions for the 
development of such a fauna have existed over a wider extent than in previous 
periods of geological history. 
In the process of sedimentation and the imbedding of the shells of this class, 
the valves have, for the most part, been separated; and the inner surface adher¬ 
ing to the rock, it becomes very difficult to ascertain the hinge structure, a 
character which should be the basis of generic distinction in fossils of this 
class. 
In far the larger proportion of the specimens of the Lamellibranchiata 
obtained from the rocks of New York, the shells have been macerated or dis¬ 
solved to such a degree that we have barely a film remaining which preserves 
the external markings, or, in some species, the characters of the interior are 
visible without having the exterior markings of the shell entirely destroj^ed. 
Many of the shells are extremely thin, hut some of the stronger forms, in cer¬ 
tain conditions of the matrix, have the shell well preserved. In these forms 
we have often well-marked casts of the interior, and rarely we may obtain 
the exterior shell in such condition as to show its entire characters. 
In the study of the species of this class of shells it is not always possible to 
determine the changes in form and expression, which may be due to physical 
influences, and in some degree also to the chemical effects operating iqjon the 
enclosing matrix. 
