Xll 
INTRODUCTION. 
tions, within the same area, is more than six hundred (600), of which about 
five hundred (500) are from the rocks above the Oriskany sandstone. 
In 1842, Mr. Conrad having resigned his position as Palaeontologist of the 
Survey, without making any final Report, the Geologists were left to their own 
resources in providing illustrations of the characteristic fossils of the forma¬ 
tions within their several districts. The final Reports of the State Geologists, 
therefore, will be found to contain illustrations of a considerable number of 
the Lamellibranchiata. 
During the interval named, and for many years subsequently, the writer, both 
personally and by the aid of his assistants, made extensive collections of this 
class of fossils in the State of New York, and to a less extent in other States. 
At a still later period, from 1856 to 1870, extensive collections were made 
preparatory for the Palaeontology of the State, through the means afforded by 
appropriations of money by the Legislature. 
In making preliminary studies of these fossils, the author found that the 
types of Mr. Conrad’s species were to a great extent inaccessible, or were dis¬ 
tributed through the collections of several institutions, and of private individuals. 
In order to secure accuracy in the identification of the described forms, he 
availed himself of the assistance of Mr. Conrad for the study and comparison 
of all the species. On three several occasions Mr. Conrad spent a considerable 
time in Albany, in the study of the collections here accumulated; and in the 
identification and determination of species, labeling one or more specimens of 
those which he recognized as typical forms of the species he had previously 
described. In this manner, and by later studies and comparisons, it is believed 
that proper identification has been made with nearly all the described forms, 
and due credit has been given in this work. 
From the time Mr. Conrad ceased his publications upon the Lamellibranchiata, 
numerous writers have published descriptions and illustrations of American 
species of this class of fossils, until at the present time the entire number of 
palaeozoic species recorded is more than twelve hundred and fifty. 
The principal authors, who have contributed to our knowledge of the subject, 
are given below in alphabetical order. To cite the date and title of the pub- 
