f 



TABULAR LIST OF THE SPECIES DESCRIBED IN THIS VOLUME, 



SHOWIITG THEIR VERTICAI« RANGE AS FAR A8 HAS BEEN OBSERVED IN THE PROGRESS OF THE WORK. 



In comparing the species of this table, and the range given, it should be borne in mind, 

 that all the groups here mentioned are subordinate parts of the lower division of the 

 palaeozoic series, constituting the equivalents of the Lower Silurian rocks of England and 

 Wales, and other parts of Europe. These subdivisions are not, therefore, to be regarded as 

 of the same importance as the greater groups or divisions, which are more definitely marked 

 at their termination and commencement ; still it is interesting to observe the important 

 changes taking place at the termination of each one, and how few are the species which 

 pass from one to the other. From the Trenton limestone upwards, the zoological affinities 

 of the strata are much greater than between this rock and those below ; for, in fact, al- 

 though it is almost impossible to define the line of separation between the Black-river 

 limestone and the Trenton limestone, yet the few species common to the two rocks impresses 

 one as remarkable, and as indicating a greater and more decided change in the organic 

 products of this period than the similar lilhological nature of the rocks would lead us to 

 believe. 



The zoological relations between the five lower members of this division are by no means 

 intimate ; the difference in the organic remains amounting almost to an entire change 

 between the termination of one, and the commencement of the next in succession. The 

 small number of species in the two lower members, however, give scarcely a fair oppor- 

 tunity of comparison with the succeeding ones where the species are more numerous ; and 

 we find no considerable number of species, till we arrive at the Chazy limestone. The 

 greater proportion of these, however, are peculiar to this rock, the few exceptions being 

 barely sufficient to link it with the succeeding strata by its organic affinities. By this 

 arrangement, and by carefully presenting the results of investigations among the organic 

 remains of very subordinate groups, geologists will be able to decide what value is to be 

 attached to this kind of evidence, even where, usually, there has not been made any 

 important distinction of age or character of strata. 



I should observe, however, in this place, that the organic affinities of these rocks may 

 be increased by further research ; as doubtless a greater number of species will be found 

 common to two or more of them, while there are only two or three instances where there 

 is a probability that those at present regarded as identical may prove distinct species. 

 Among these are the Illtenus and Isotelus of the Chazy limestone, which are apparently 

 identical with those of the Trenton limestone. 



The Utica slate has been continued as a separate formation in this table, though the 

 species peculiar to it are few. Its relative value and importance will be readily apprehended 

 by an inspection of the table. 



[ PaljEontolooy.] 41 



•-V ? 



