X1Y 
INTRODUCTION. 
was distributed to the public * Since that time, the author has had no 
opportunity of completing the printing according to the original plan of the 
work. 
This Preliminary Notice was originally prepared for incorporation in the 
23d Report of the State Museum of Natural History, but it was published in 
advance, as a separate pamphlet, and has never been incorporated in any Report 
of the State Museum. The species there described have been included in the 
present and preceding volume with proper reference to the original publication. 
The Preliminary Notice of the Lamellibranchiata No. 1, including the 
Monomyaria, was communicated with the 34th Report of the State Museum 
of Natural History, but was only published in the 35th Report, issued in 1884. 
This paper, with plates giving generic illustrations, has been published in a 
separate form, making a pamphlet of about 200 pages with five lithographed 
plates of generic illustrations. 
There are very few forms of Lamellibranchiata known in the primordial zone, 
nor until we reach the Chazy and Trenton periods are the fossils of this class 
found in any considerable number. In volume I of the Paleontology of New 
York, there are but two species described from rocks below the Trenton lime¬ 
stone. Twenty-six species were described from that formation, and in the 
Utica slate and Hudson-river group, fourteen species were recognized. Later 
collections have largely increased the number, and it is not improbable that a 
study of the material since accumulated from the same localities will show 
double the number then known. So far as observed, the individuals of species 
in the Trenton limestone are not abundant, a few forms only having been 
collected in considerable numbers. 
The species are more common in the limestones where there is consider¬ 
able argillaceous matter, either intimately mingled with the calcareous mass, 
or when the calcareous layers are separated by argillaceous seams; and the 
fossils are usually casts of the interior, the shells being rarely preserved. 
In the Hudson-river group, within New York, the species of this class, while 
* The printing was discontinued at this point, page 96, owing to the burning of the printing office with 
all the matter in type except the stereotyped plates. 
