Chap. X.] 
UPPEE SILUEIAN FOSSILS. 
215 
CHAPTER X. 
FOSSILS OF THE UPPEE SILUEIAN EOCKS. 
The reader has already been told that many species of fossils, once sup- 
posed to be peculiar respectively to the Lower or Upper Silurian rocks, are 
now ascertained to be common to both, and necessarily to the intermediate 
and connecting group. 
This datum is the result of the researches of various geologists and 
palaeontologists, whether in the region first explored or in tracts of far 
greater extent which have recently been paralleled with it. Similar results 
have, in truth, invariably followed from a full and broad development of 
the natural geological groups of the Secondary and Tertiary strata, which 
were described and classified before the older rocks of which we now treat 
had been brought into order, or even into notice. Thus, for example, 
when the different members of the Oolitic formations reposing on the Lias 
were studied in one tract only, as on the eastern coast of Yorkshire, they 
were seen to be there composed of a series of zones, each of which is 
sharply separated from the contiguous deposits by fossils confined to it. 
On tracing, however, the same strata to remote distances, certain remains, 
which were once viewed as typical of one member only, were found to be 
common to several subformations, thus combining the whole in one natural 
system — the Oolitic or Jurassic. 
This is just what has happened in the Lower Palaeozoic rocks, now that 
the inferior and superior masses, and the chief formations and subdivisions 
of Siluria have been ascertained by my cotemporaries to occupy nearly all 
North and South Wales, large tracts of Cumberland, "Westmoreland, and 
Lancashire, great regions in Scotland and Ireland, and various parts of 
Europe, America, and Australia. 
In short, the two chief divisions, which, from a general similarity, were 
originally grouped together, have been demonstrated to constitute a natural 
system, through a community of organic remains. For, even if the con- 
tents of the intermediate middle zone of Llandovery rocks be abstracted 
from the estimate, still there are many species (from fifty to sixty) which 
range from the Llandeilo and Caradoc into the Wenlock and Ludlow rocks. 
The vertical range of all these fossils through the chief Silurian deposits 
is given at the end of the volume, in a Table prepared by Mr. Salter, with 
the aid of Professor Morris, and subsequently revised and much augmented 
by Mr. Etheridge. 
To some of the most striking of these remains which pervade the whole 
