Chap. IX.] 
FOSSILS OF THE LLANDOVEKY EOCKS. 
207 
fossiliferous strata known. It is, indeed, a remarkable fact that the most 
sedulous research in many parts of the world has failed to discover the 
trace of any vertebrated animal in the lower division of the Silurian 
system. All the marine animals (amounting to many hundred, perhaps 
even to a thousand species) already known belong to the invertebrated 
classes, no Fish having yet been observed. This observation applies 
also to the Upper Silurian rocks of Britain, with the exception of their 
highest bands, — a fact formerly dwelt upon in the ' Silurian System.' 
Having thus referred to the figures first published in my original work, 
and now reproduced at the end of this volume, as well as to other types of 
Lower Silurian age which have been subsequently discovered, we now 
pass on to consider the assemblage of fossil animals specially characteristic 
of the Llandovery rocks — the formation which connects the Lower and 
Upper Silurian divisions. 
Fossils of the Llandovery Rocks. — The principal organic remains belonging to 
this zone/which I previously regarded as the upper portion of the Lower Silurian 
rocks, are now eliminated from the catalogues through which they were diffused 
in the ' Silurian System ' and in the first edition of this work, and are here 
brought together. (See Pis. VIII.-XI. ; and Foss. 15, p. 90.) 
This important and varied group, which is also found to intervene between 
the much greater masses of Lower and Upper Silurian rocks in Russia, Scandi- 
navia, and America, is nowhere more largely developed than in the British Isles. 
Though linked on to the Lower Silurian by several typical forms, the fauna of 
these rocks has, on the whole, a peculiar facies ; and many of the species in the 
superior division of the series are Upper Silurian. The great number of TriJo- 
bites we have been considering in the earlier part of this Chapter have disap- 
peared, and a few rare species of those ancient Crustaceans, including one or two 
that are specially distinctive of this intermediate series, are mingled with others 
which become plentiful in the Upper Silurian rocks. The prevailing Corals, 
Brachiopods, and Univalves differ but little from those of the inferior strata ; 
there being several typical Shells which continued to live on. On the other 
hand, certain forms which have not been found in the Llandeilo and Cara- 
doc formations give a marked impress to this group. Some of these differences 
may, no doubt, be due to the gravelly and sandy nature of the strata, com- 
pared with the more argillaceous materials of the Lower Silurian rocks ; but 
this explanation will not suffice when the arenaceous members of both groups 
are put in comparison. 
Though Graptolites are rare, G. priodon is present, a species which ranges 
from the true Lower Silurian up to the Ludlow rocks. The Corals are by no 
means few in number; and many of them — such as the Chain-coral (Halysites), 
the various species of Favosites and Heliolites, Syringopora, &c. — are the same 
as those known in the Lower and Upper Silurian. Favosites Gotlandicus and 
Heliolites interstinctus are common everywhere, and these species seem to have 
been indifferent (as very common species usually are) to the nature of the sea- 
bottom on which they lived. Some new species are added, both of the Mille- 
pore (Zoantharia tabulata) and Cup-coral (Rugosa*) groups, which will be 
noticed in their proper places. 
* This group of the Actinozoa will be again referred to in the sequel. 
