INTRODUCTION. 



xy 



fewer than in the Trenton limestone, are more numerous as individuals, and 

 make a much more conspicuous feature of the formation than they do in the 

 Trenton limestone. 



In the Trenton limestone, at Ottawa, in Ontario, the shells of the Lamelli- 

 branchiata are replaced by silica. This condition of the fossil enabled Mr. 

 Billings to remove the calcareous matter by acid and determine the hinge- 

 structure of several forms which were before unknown. In the Canadian 

 Reports, this author has recognized a single species from the Calciferous sand- 

 stone and four species from the Chazy limestone. From the Lower Silurian 

 (as then understood), he has published thirty-two new species, or a number 

 nearly equal to all which had before been known to the public from the same 

 rocks. He has likewise recognized, in the Canadian rocks, a large proportion of 

 the species known in the New York formations of the same age. 



The Trenton and lower limestones in their westerly extension are not more 

 prolific in species of Lamellibranchiata than the same rocks to the eastward; 

 but many new forms appear in the western localities. In this respect, how- 

 ever, the Hudson-river group forms a striking contrast with the same forma- 

 tion in the east. With the increase of calcareous matter in its composition, as 

 traced in a westerly and north-westerly direction, the fauna becomes more 

 abundant, and finally, in its greatest development, is more prolific in some 

 classes of invertebrate fossils than any other of the palaeozoic formations. 

 With this general development the Lamellibranchiata keep pace, and the 

 number of species described from this group in Ohio, Kentucky and Tennessee, 

 and other western localities, far exceeds all that have been described from all 

 the Eastern States. The entire number of species from this group, enumerated 

 in Miller's Catalogue, is eighty-seven. 



In the succeeding sedimentary formations of the Medina sandstone and Clin- 

 ton groups of New York there are few species of this class of fossils ; and the 

 number in the calcareous measures constituting the Niagara group is scarcely 

 greater. In the Palaeontology of New York, vol. II, twenty-seven species are 

 described, of which two are from the Medina sandstone and nine from the 

 Clinton group, leaving the remainder (including those from the Coralline lime- 



