INTRODUCTION. XXxl 
given below. In many parts of the Cambrian Series they are found in millions: and 
they produce a very characteristic impress on its Fauna. But not so much as a single 
unequivocal fragment of any one of them is to be found in those parts of the Cambridge 
collection which are derived from beds=below the May Hill sandstone, Negative facts cannot 
stand a moment against positive. This we all allow: but if negative facts be honestly and 
laboriously stated, they may, at least, prove that certain positive facts (such as the six 
examples above quoted of species common to Cambrian and Silurian rocks) are very rare and 
exceptional cases. Such cases are not to be received but on evidence that is unequivocal ; 
viz. the production of the very species upon which the exceptional lists have been formed. 
Now I was informed by Professor M*Coy (in a note which is now before me) that he had 
applied, at the Museum of Economical Geology, for a sight of five, out of six, of the very 
species above enumerated, and that not one of them was shewn to him, or found in the 
Museum, under conditions which confirmed the assertion that they were common to Llan- 
deilo, Wenlock, or Ludlow rocks. As to one of the six species (Orthis Actoniw) I myself 
applied to Professor Forbes for his authority. He replied that he could not give his sanction 
for this species as a type common to the (so-called) Upper and Lower Silurian rocks—that 
he had nothing but a field memorandum respecting it, and that he might very easily have 
mistaken the species, or mistaken the rock from which it was obtained. 
Taking Sir R. I. Murchison’s list, as published in the Appendix to his “Siluria,” and 
without any deduction whatsoever, it does not give us much more than ten per cent. of species 
common to true Cambrian and true Silurian British rocks. But when we have struck out 
from this common list, (1) all those species which range beyond the limits of the Lower 
Paleozoic Division, into the Devonian and Carboniferous groups; (2) all species of doubtful 
authority ; (3) all species of which the geological place is not established on good physical 
evidence; (4) all species which however abundant in the /ower series disappear in the upper— 
either altogether, or ascend into it through so very limited a space above the line of demarcation 
as to produce no general impress on the upper Fauna;—when all this has been done, I 
have no doubt that the number of common species will fall considerably below ten per cent.: 
perhaps as low as the percentage shewn by the Palzeozoic series of North America or Bohemia; 
but not so low as the per centage shewn by the Cambrian and Silurian rocks of the North of 
England. 
Further Remarks on the Cambrian Series, and on the Nomenclature of its subordinate 
Groups. 
On returning to the Cambrian series, I know well how impossible it is to describe 
it in any adequate terms within the limits of this Postscript: but I may give a short 
summary of some of the leading facts connected with its history which could not be 
noticed in the Tabular View. irsély. In extent, in complexity of structure, and in gran- 
deur of development, it rises far above all other collective Paleozoic groups; and in 
Th Ht 
