26 INTRODUCTION. 
what arenaceous, with courses of impure limestone. Its fossils, particularly the Brachio- 
poda, are so characteristic, that, whether you meet with them on the slopes of Caer 
Caradoc, where they were first described, or at Bala and Snowdon, they leave no doubt 
whatever of the age of the rock to which they belong. For example, Stphonotreta micula, 
Orthis alata, O. vespertilio, and O. Actoniea, with Strophomena expansa, Leptena sericea, 
and Z. tenuicincta occur abundantly ; added to which, other and important forms of life 
come in, such as Glyptocrinus, Spheronites, and Pleurocystites, among the true Crinoids, 
with Paleaster, &c., in the group of the Asteriadee. Homalonotus bisulcatus, Trinucleus 
seticornis, Cybele verrucosa, and Asaphus Powisit, illustrate a rich group of Trilobites ; 
whilst numerous Cephalopods of the genera Cyrtoceras, Orthoceras, and Lituites, occur, 
all equally important, as determining and fixing the age and position of these Caradoc or 
Bala Rocks. 
The rocks hitherto enumerated constitute the Lower Silurian division of my classifica- 
tion, in which, as before remarked, no trace of a vertebrated animal has been discovered. 
These rocks are largely associated with masses of igneous origin, both cotemporaneous 
and intrusive; and to comprehend their relations, as well as their various dislocations 
and overlaps, the Map of the Government Survey, and particularly the recently published 
work of Professor Ramsay above mentioned, must be consulted. The total thickness of 
these rocks has been estimated at about 17,000 feet. 
In the earliest years, when I was preparing the first Silurian Classification, I simply 
divided the system into Lower and Upper Silurian rocks; but in the sequel, and by close 
examination of the organic remains, it was found that some of the strata, having more or 
less of an arenaceous character, which formed the summit of the Lower Silurian rocks of 
my earliest tabular arrangement, were in reality characterised by many new species, and 
deserved a separate name. ‘These strata I named “ Llandovery Rocks,” from their full 
development near the town of that name in South Wales. Again, in other tracts, par- 
ticularly at May Hill, in Gloucestershire, some fossils were found to pass upwards into 
younger beds, which, in their turn, were surrounded by the true base of my original 
“Upper Silurian” Rocks. Hence, in the last edition of ‘ Siluria’ (1859), I adopted the term 
Llandovery Rocks,” to mark this intermediate group, the lowest beds of which graduate 
down by their fossils into the Caradoc Sandstone, and the highest of which are connected 
~ with the Woolhope Beds, or base of the Wenlock Formation. 
The Brachiopods drawn and described by Mr. Davidson unambiguously prove that 
these Llandovery strata form a central link, thus uniting more closely together the 
Lower and Upper Silurian Rocks. 
As illustrative of the evidence and correctness of this view, and the stratigraphical 
value of the Brachiopoda as a group of fossils to be used for this purpose, I merely 
offer the names of a few genera and species, noticing first those forms which do not occur 
below the Llandovery rocks. There are Pentamerus oblongus, Rhynchonella decempli- 
cata, Rh. navicula, Rh. obtusiplicata, &c., Lingula crumena, Athyris (Meristella) tumida, 
