XXX INTRODUCTION. 
fortified and defined by a co-ordinate weight of fossil evidence. Nor does it much matter 
by what names we call these several subdivisions. Whether each is to be called a System or a 
Series, seems, at first sight, rather a question of taste than of science. I think, however, that 
the facts just stated do prove that the word Series is a more correct description of the 
Paleozoic subdivisions than the word System—among other reasons because it is a less definite 
term, and admits of a broader margin: and to this reason we may now add, with perfect 
certainty, that the abuse of the word System has been a drag-chain on British Paleozoic 
Geology, and has led to many and great mistakes, both in classification and nomenclature. 
Sizxthly. To the previous remarks I may add, that if the succession of our geological 
deposits were physically complete, we might, with proper caution, apply the percentage theory 
of Sir Charles Lyell to their limitation and nomenclature, by counting the well-ascertained 
species in each successive group. But in the actual condition of our paleontological series 
we cannot follow this rule, universally, without introducing the elements of confusion. Thus 
on the percentage theory, we should unquestionably be led to give a false date to the Red 
Crag of Suffolk; for this simple reason, that a great number of its fossils have been mecha- 
nically drifted out of an older deposit, the Coraline Crag—a fact first clearly pointed out 
by Mr Charlesworth. 
There may not be a similar example in the whole British Paleozoic Series: but assuredly 
it is not improbable that among the coarser and more mechanical Palzozoic deposits (such as 
the Red Sandstone which overlaps the Carboniferous rocks and forms the base of the Permian 
Series; or such as the coarse mechanical beds of the May Hill sandstone, which, in like manner, 
overlap the Cambrian rocks, and form the base of the Silurian Series) there may be some 
examples of species which have drifted out of the rocks of an earlier date. On this ground, 
while we are making a comparative estimate of the Cambrian and Silurian Faunas, we can 
only count at a very low value such species as abound in the lower, and appear very rarely 
in the upper division; and appear there only within a very little distance of the line of 
demarcation between the two. To count such species of the same numerical value in the 
two Faunas, would be as gross an instance of miscalculation as could well be imagined. 
Seventhly. Without dwelling on cases of ¢mmediate ambiguity, we may affirm generally— 
that when we profess to give a comparative estimate of the Fauna of any two Paleozoic 
groups, we have no right to overlook the question, whether certain species belong to a 
prevailing type in one of the groups; or, on the contrary, are rare (and perhaps doubtful) 
exceptions. I will take, for example, six species which are given by the author of “ Siluria” 
as common to the Llandeilo, Wenlock, or Ludlow formations; viz. Zentaculites annulatus, 
Petraia elongata, Trinucleus concentricus, Lepteena sericea, Orthis Actoniee, Orthis vesper- 
tilio*. This list might be largely increased; but I select these six species for a reason 
* If the reader turn to the following pages of this Volume where these species are described, he will see how largely 
some of them have been collected. But the published lists, ample as they are, give a very inadequate notion of the mass of 
materials that was submitted to Professor M°Coy during the years that he laboured in our Museum. Hundreds (or I might 
say thousands) of specimens were broken up by him with a view of determining the nature of the hinge, or some character 
(occasionally microscopic) which was important to the determination of the seyeral species. 
