INTRODUCTION. XXI 



The Cephalopods 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 Birdscyc limestones, but are the most conspicuous forms in the 

 Black-river and Trenton, both of which may with propriety be termed " Orthoccratite 

 limestones." The lower division of the system is pre-eminently the era of Cephalopods of 

 the order Orthocerata ; and even throughout the entire palfeozoic 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 Trilobites, 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 Illcenus 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 palaeozoic strata. 



Thus far, no remains of Fishes, 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 paleozoic 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 Iiold 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, 

 Avliere the terebratuloid forms increase in the higher rocks, and predominate greatly over 

 Orlhis and Leptcena. The Ortlioceratites 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 Corals, the Cheetvtes 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 dilVerent in their habit and mode of 

 growth as to appear more nearly allied with Favosites. 



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