vi ADVERTISEMENT TO SECOND FASCICULUS. 
mistaken; for it is the equivalent of the Bala limestone which is in the centre of a true 
Cambrian group; and to this Cambrian group it was made superior, and described as united 
in one group with the Caradoc sandstone. 
Let the Cambrian rocks be called Cambrian, and the Silurian rocks be called Silurian, 
as first agreed upon; still there will remain a considerable difficulty presented by the Caradoc 
sandstone. In the former Advertisement I stated that this group formed the physical base 
of the true over-lying Silurian series, while paleontologically it was more connected 
with the Cambrian groups. Hence, on paleontological grounds I have ranged the Caradoc 
sandstone with the Cambrian series, as seen in the above Tabular View. But if all the rocks 
now called Caradoc sandstone be of the same age, then the evidence is in conflict; for the 
fossils of Horderly are in very near agreement with those of Bala, while the fossils of May Hill 
are in near agreement with those of the Wenlock group. Perhaps these discrepancies may be 
soon removed by a new adjustment of the Caradoc group. Meanwhile, that interpre- 
tation of the facts which is most in conflict with the conclusions I am advocating, would 
merely prove, that during a period of movement and change of conditions which introduced 
the true and well-defined Silurian deposits, species very characteristic of the Silurian 
groups were flourishing in one part of the sea, contemporaneously with species in other 
parts of the sea which were equally characteristic of the upper Cambrian groups. 
Leaving the discussion of such questions as unfit for these introductory remarks, I call 
the reader’s attention to the wide difference exhibited between the fossils of the Cam- 
brian and Silurian series. 
On counting over the lists, including a few supposed varieties, I find 304 species given to 
the Cambrian Series, and 262 to the Silurian, while 85 are common to both. This gives about 
15 per cent. of the whole series common to the Cambrian and Silurian rocks. But this per 
centage is too large; for many doubtful cases of common species were recorded in counting 
up the lists. Are the true Silurian rocks palzontologically more separated from the Devonian, 
and the Devonian from the Carboniferous rocks? I greatly doubt it. It is obvious that the 
greatest interchange of species must be near the plane which marks the separation of one 
group from another. In the fine list of fossils from the Upper Ludlow group of Westmoreland 
there are scarcely more than three or four species found in the lists of the Coniston fossils: 
unless it be species that ranges through nearly the whole palaozoic system. 
I conclude this Introduction by noticing one or two of the more striking additions made 
by Professor M°Coy to our paleontological knowledge. 
The discovery of the peculiar diaphragm at the base of the cells in the Graptolites (as in 
the recent Sertularia) and other points in their structure, fixes their place among the 
Hydroid polyps with certainty—completely refuting the generally-received notion of their 
being allied to the Sea-pens (Virgularia), as well as the opinion recently revived by Geinitz 
of their being Cephalopods, and that given in the preface to the “Geology of Russia,” of their 
being sea-weeds. 
The fossil identified by former observers as an Ichthyodorulite, from the Bala limestone, 
