TABULAR LIST OF THE SPECIES DESCRIBED IN THIS VOLUME, 
SHOWING THEIR VERTICAL RANGE AS FAR AS 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 
paleozoic 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 lithological 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 I/enus 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. 
| PaLaonrToLocy.] 41 
