INTRODUCTION. xxi 
The CepyaLopops appear among the earliest organisms, two species occurring in the 
Calciferous sandstone. Though appearing in all the succeeding strata, they are not nu- 
merous in the Chazy and Birdseye limestones, but are the most conspicuous forms in the 
Black-river and Trenton, both of which may with propriety be termed ‘ Orthoceratite 
limestones.”’ The lower division of the system is pre-eminently the era of Cephalopods of 
the order Orthocerata ; and even throughout the entire paleozoic series in the United States, 
no subsequent formation affords so great a development in any of the other families of this 
class. The several new genera of these fossils are founded on peculiar characteristics, 
indicating, indeed, in one instance, even more than a mere generic distinction. 
We have a sufficient number of Tritosites, identical with those of the Lower Silurian 
rocks of Europe, to institute a comparison of the conditions of the ancient ocean in both 
hemispheres. That remarkable and characteristic Lower Silurian form, Trinucleus, is among 
the most common; while Ilenus and Isotelus, no less characteristic, are obtained in the 
earliest limestone. Several new species have been added; while others, hitherto but im- 
perfectly understood, have been more fully illustrated. It is not a little remarkable, that at 
this early period we have the representatives of nearly all the important genera of Tri- 
lobites known in our palzozoic strata. 
Thus far, no remains of Fisues, or other animals of a higher order, have been dis- 
covered ; though we find them almost at the commencement of the next division, and in 
all the succeeding palzozoic strata. 
The results of these investigations in the lower formations, though necessarily very 
imperfect, have nevertheless shown a comparatively equal or proportionate development 
among the different classes ; and though certain forms appear often to hold a predominance, 
it is frequently due to the development of a greater number of individuals, rather than the 
number of species. In other cases, the species of certain genera are more numerous, while 
in a succeeding rock the order may be reversed. This is true in regard to the Brachiopods, 
where the terebratuloid forms increase in the higher rocks, and predominate greatly over 
Orthis and Leptena. The Orthoceratites diminish in number of species, and individual 
preponderance ; while forms like Cyrtoceras and Goniatites, which are rare in lower rocks, 
become the predominating genera of this class in the higher strata. 
Among the Corats, the Chetetes of the Lower Silurian strata gives place to Favosites, 
which flourishes in abundance. All the Cyathophyllidea of the lower division of the system, 
having turbinate forms, are destitute of transverse or horizontal septa; * and the first 
* The two species, Columnaria alveolata and Favistella stellata, have transverse as well as vertical septa; but 
these forms, though regarded as belonging to the same family of Corals, are so different in their habit and mode of 
growth as to appear more nearly allied with Favosites. 
