XIY 



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 printiriff w«h disoontinued at this point, page 96, owing to the burning of the printing office with 

 nil the matter in type ex<^ept the atereotyped plat«a. 



