XV111 
INTRODUCTION. 
imestone, becomes very abundant and varied in form in the Hamilton group, 
reappearing in considerable numbers, and acquiring very extravagant forms 
in the Chemung group. 
The genus Grammysia, so abundant and characteristic in the Hamilton group, 
is far less conspicuous in the Chemung group, gradually losing its characteristic 
features to such a degree that it becomes a question with regard to some forms 
whether they shall be referred to Grammysia or Allorisma. 
At the base of the Upper Helderberg group, the Schoharie grit carries a few 
species of this class of fossils which are peculiar to that rock. A considerable 
number of species make their appearance for the first time in the Corniferous 
limestone; and some of these are continued into the Hamilton group. A few 
species in this rock are remarkable for their persistence. The Aviculopecten 
princeps of Conrad is known in this horizon in New York, Ontario, Ohio and 
Indiana. 
The Hamilton group in its western extension becomes a calcareous forma¬ 
tion, and is represented in Wisconsin, at the Falls of the Ohio, and at other 
places, by a magnesian limestone, with more or less shaly matter interspersed 
throughout the mass, or intercalated in thin laminae. At the Falls of the Ohio, 
the Hydraulic beds, which are equivalent or representative of the Hamilton 
group, contain at least twenty species of Lamellibranchiata, ten of which are 
likewise common to the Hamilton group in New York. 
On the Mississippi river, the Hamilton group is represented by a limestone 
carrying many of the characteristic fossils (especially Brachiopoda) of the 
formation in its eastern development. 
The Chemung group in its lower members is more restricted in its geograph¬ 
ical distribution than the Hamilton group. Its central and upper members, 
have a wider expansion than the lower beds, and everywhere carry a peculiar 
and characteristic Lamellibranchiate fauna. Some of the genera common to 
the Hamilton group disappear almost entirely, while many new forms come in 
above the Portage; and several genera first appearing in lower horizons, here 
reach their greatest development in number of species. 
In the lower members of the group, as at Ithaca, and adjacent localities, a 
