viii PREFACE. 



the writer, assisted by the drivughtsman, Mr. Geo. B. Simpson. The results of 

 that investigation were publisJied in the Thirty-second Report of the State 

 Museum of Natural History, and the plates having already been lithographed 

 at that time, the species were referred to plate and figure as the same appear 

 in the present volume. These plates with explanations of the figures were 

 communicated with the Report of the State Geologist for 1882, and were pub- 

 lished in photo-lithography in 1883. In the same report there were also 

 published ten plates illustrating the Fenestellidae and other forms of the Upper 

 Helderberg group. 



The greater part of the species described and illustrated in this volume have 

 been described in the Annual Reports of the State Museum, the Reports of the 

 State Geologist, and in the Transactions of the Albany Institute. In the 

 Reports of the State Geologist for 1882 and 1884, published in 1883 and 1885, 

 was printed a discussion upon the mode of growth and relations of the 

 FenestellidsB, which will be completed and published at a future time. 



The total number of species from the Lower Helderberg group, described in 

 this volume is one hundred and three, of which two species are not illustrated.* 

 The immber of species described from the Upper Helderberg is one hundred 

 and fifty-four, of which thirteen .are not illustrated. From the Hamilton group 

 there are one hundred and twenty-one species described, of which thirty-five 

 are not illustrated in this volume, but it is hoped that they may soon be pub- 

 lished through some other medium. 



The plates devoted to the illustration of the Lower Helderberg forms, 

 including the Corals proper, are from i to xxiii a. The species of the Upper 

 Helderberg group are illustrated by plates xxv to liv, the FenestellidaB alone 

 occupying twenty plates. The Bryozoa and Bryozo()id forms of the Hamilton 

 group, which are illustrated, occupy plates Iv to Ixvi inclusive. 



There are some interesting facts connected with the Geological and Geo- 

 graphical distribution of these forms of life, but the space at my command will 

 not admit of a full discussion of this subject. 



* The com|>Bt«tlvcly few formo of Coi-ala proper, in the Lower HeMerlmrg' proup, i-eiidered their intro- 

 duction in thin volume practicatile ; but fi-oin the Upper Helilei'berg and Hamilton groups, no attempt has 

 bMo made to introduce them. 



