XVUl 



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 AUorisma. 



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 Aviculopeden 

 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 



