28 “ INTRODUCTION. 
usher us into the first great Ichthyic period, the Devomian or Old Red Sandstone, so 
well marked by its Fossil Fishes in Russia, Britain, and North America. 
No zoological feature in the Upper Silurian rocks is more striking than the great 
increase and profusion of Cephalopods, many of them of great size, which appear in strata 
of the age immediately antecedent to the dawn of vertebrated life. A grand illustration 
of this phenomenon has already appeared in the second volume of the ‘ Systeme Silurien 
de Bohéme,’ by M. Barrande, who has already figured and described,—Goniatites, 17 
species ; T'rochoceras, 44 sp.; Nautilus, 7 sp.; Gyroceras, 7 sp.; Hercoceras, 2 sp. ; 
Lituites, 7 sp.; Phragmoceras, 32 sp.; Gomphoceras, 70 sp. ; Ascoceras, 15 sp. ; in all, 
202 species, or distinct varieties, as occurring in the rich Silurian Basin of Prague ; and 
the greater number of them: named, as well as figured and described, by Barrande. I 
learn from my distinguished friend that when his grand and classical work is completed, 
no less than 3000 species of fossils will have been described from the limited Silurian 
Basin of Bohemia alone ! 
It is generally believed, and I embrace the view, that this group of large and straight- 
chambered Cephalopods preyed upon the numerous lower tribes of marine life during the 
long Silurian era; and, as relates to Great Britain, it is curious to notice, that with the 
advent and prevalence of Fishes in the Devonian era, the gigantic Cephalopods diminished 
in number and in size. From their first appearance to the present day, the numbers of 
genera and species of Fishes, which are, we know, the most predacious animals of the sea, 
have gone on increasing, whilst the more voracious and large Cephalopods have become 
exceedingly rare. 
In the progression of life, as above sketched out, the Silurian System is characterised 
throughout by certain Brachiopods, particularly Lingulide, Orthide, Pentameri, &c., 
which everywhere prevail; also by the pennate Zoophyte called Graptolite, which, as 
before said, has not been found in younger rocks; by Trilobites of certain genera, 
nearly all of which are peculiar to it; and by Cephalopoda and Echinodermata, of 
forms never found in any overlying or younger deposit. 
Now, in all this immeasurably long series of geological deposits, beginning with the 
dawn of Molluscan life, the Brachiopods are the most abundant, persistent, and reliable 
types of life by which geologists recognise the different formations. Hence, it has given 
me the truest satisfaction, that my eminent friend, Mr. Thomas Davidson, should have at 
last completed his long labour of love by terminating his description of the Paleozoic 
Brachiopods of Britain with so skilful and elaborate a delineation and description of all the 
species of this class known in the Silurian Rocks. Furnished as he has been from many 
quarters with fossils of the several formations constituting the Silurian System, it was 
peculiarly gratifying to me, as Director-General of the Geological Survey of the United 
Kingdom, to place under his examination all the Silurian and other Paleozoic Brachiopods 
in the cases of the Government Geological Museum in Jermyn Street, illustrating the 
Survey.. It is also important to mention that, having considered the Jermyn Street 
