PALEOZOIC FOSSILS. 
405 
Tlie Cambrian rocks in this country are enormously developed, 
probably being not less than five miles in thickness ; they rest uncon- 
formably on the still more ancient Archrean rocks, which have lost all 
traces of the organisms that in all probability lived during the period 
of their deposition, so that the relics of the organized life of the 
Cambrian period are the earliest of which we have cognizance in this 
country, but we find therein well organized representatives of all the 
classes of mollusca, and we must, therefore, look immeasurable ages 
further back for the ancestral form from which they originally 
diverged, all traces of which have been obliterated by the excessive 
denudation and changes which the earlier strata have undergone. 
No representatives of land and freshwater genera are recognized in 
Cambrian strata, but, as those groups are directly derived from 
marine species, it is probable that the earlier forms would retain for 
a time a marine facies and not be readily separated from the truly 
marine species from which they originated. 
The Devonian rocks are well distributed over the British Isles, and, 
with the exception of strata found in Devon and Cornwall, are probably 
mainly of fresh-water origin and known as the Old Red Sandstone, 
the colouring of the strata being due to iron oxides. This formation is 
remarkable for the discovery at St. John’s, New Brunswick, of the 
earliest known Pulmonate, Pupa primcvva, and in this country for 
the appearance of 
Archanodon jukesi Baily, 
a species scarcely distinguishable externally from the ordinary Ano- 
donta of our own day, and the first undoubted representative of our 
fresh-water fauna, which has been found near Cork, at Kiltorkan 
and Tallow Bridge in county Waterford; near Caerleon in Monmouth, 
and in Northumberland. 
The Carboniferous system is so called because it contains the chief 
coal deposits, known as the Coal measures, which are as much as 
10,000 feet thick in South Wales. Conchologically, this deposit is 
remarkable for the discoveiy in the South .Joggins Coal-field, Nova 
Scotia, of a number of terrestrial Pulmonate species referable to 
various genera, and in this country for the appearance of a wealth of 
species of primitive Unionidie and Mytilidse, classified as Carbonicola, 
Anthracomya, and Naiadites. 
The Carbonicola awA Anthracomya are Unioniform genera, differing 
from the modern Unioiies in the dorsal and posterior position of the 
