PREFACE. ix 



In the Lower Helderberg group the collections of Bryozoa have been chiefly 

 made from the outcrop along the north-eastern base of the Helderberg range, 

 in the neighborhood of Clarksville, Albany county. A less number of forms 

 have come from Schoharie, about twenty miles to the westward of the first 

 locality. Bryozoans are rarely, if at all, seen in this group to the west of 

 Schoharie county. A smaller number of specimens have been obtained at the 

 outcrop of the formation on Catskill creek, and a few at Becraft's mountain, 

 near Hudson. Beyond these points to the southward few forms are known. 



In tlie Upper Helderberg group, the Bryozoa and Bryozooid forms are com- 

 paratively rare in the eastern part of the State of New York. It is not until 

 we reach the central portion of the State that these fossils become conspicuous 

 in a few localities, and their occurrence in these places seems due to the local 

 development of certain sediments, which form no important feature in the entire 

 formation. The best known localities are Onondaga Valley, Caledonia and Le 

 Roy, and from the latter point westward they become more abundant. In Erie 

 county we meet for the first time, so far as observation has been made, a 

 siliceous layer near the base of the formation, which is charged with Bryozoa, 

 especially of the family Fenestellidae. This deposit is much more fully devel- 

 oped on the west of the Niagara river, containing, in addition to the Bryozoa, 

 Trilobiies, Brachiopoda, etc. 



The most prolific locality of the Fenestellidae at present known occurs in the 

 limestone of the Upper Helderberg group, at the Falls of the Ohio river, 

 where these fossils have been silicified and the inclosing calcareous matrix has 

 been removed by solution. 



Notwithstanding the comparative dearth of Bryozoa in the Upper Helderberg 

 group in eastern and central New York, the Corals proper abound throughout 

 the entire extent of the limestone from the Hudson valley to the Niagara river. 



In the Hamilton group all forms of Bryozoa are extremely rare in the sedi- 

 mentary deposits of the eastern part of the State. As the shales become cal- 

 careous in the central parts of the State, these organisms increase in number, 

 and become abundant in nearly all localities from Seneca lake westward, the 

 most prolific region being that portion of country extending from the Genesee 

 valley to Lake Erie. 



