Early 'Triiobites of the Cambrian (Rocks — Matthew. 3 
The lower series also contains worm tracks of ordinary size 
{^Helminthites') and the double worm burrows named by J. W. 
Salter Arenicolites. It has also yielded an example of a large 
thin-shelled brachiopod similar to Llngiila ( /) monolifera of 
the Eophyton sandstone of Sweden. 
This series of Cambrian rocks is about 1200 feet thick. 
B. AcadiaJi series. 
The series above described is overlaid unconformably by the 
Acadian or St. John series which has an abundant trilobite fauna 
in its lower part — Stage (or Division) i. All the genera of 
triiobites of this stage are known in the Cambrian system in 
Europe, and are the following: Paradoxides, Ellipsocephalus, 
Solenopleura, Ptychoparia, Liostracus, Agraulos, Ctenocephalus, 
Conocoryphe, Microdiscus and Agnostus. 
Triiobites in the comparative ra2:)idity with which they were 
introduced in the several layers of the paheozoic strata, and 
the short period during which the species remained in existence, 
serve the same purpose in discriminating the members of the 
ancient geological systems, as in the Tertiary strata is fulfilled 
by the mammals. This peculiarity of these ancient crustaceans 
is well shown in the close parallelism of the species on both 
sides of the Atlantic, and the rapid exchange of one species for 
another in passing from older to younger beds of the Cambrian 
system. In the lower part of this system (^. g. stage i ) we do 
not find many identical species in Scandinavia and Acadia; but 
there are a large number of representative species; these do 
not occur in exactly the same order in the two countries, for 
some forms that show themselves first in Europe, as Paradoxi- 
des tessiui^ appear later here {^P. abetiacits)', SLXid others that 
are foixnd among the oldest here P. eteminicus {cf.P. rugti- 
losus) make their appearance as constituents of a later sub- fauna 
in Europe. There are some sub-genera of triiobites that are 
peculiar to this side of the Atlantic, but the genera are alike on 
both sides. 
An apparent exception to this rule is Paradoxides ( P) kjeru/ji 
of Scandinavia, of the oldest trilobite layer in that country; but 
the apparent absence of such a form on this side of the sea is 
probably due to the scarcity of organic remains in the corres- 
ponding beds in Acadia. 
