Vlll 
PREFACE. 
the writer, assisted by the draughtsman, Mr. Geo. B. Simpson. The results of 
that investigation were published 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 Fenestellidse 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 
Fenestellidse, 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 number 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 Fenestellidas alone 
occupying twenty plates. The Bryozoa and Bryozooid forms of the Hamilton 
group, which are illustrated, occupy plates lv to lxvi 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 comparatively few forms of Corals'proper, in the Lower Helderberg- group, rendered their intro¬ 
duction in this volume practicable ; but from the Upper Helderberg and Hamilton groups, no attempt has 
been made to introduce them. 
