480 
ELEMENTS OF GEOLOGY. 
on both sides of the Atlantic is between 30 and 40 per cent.; a 
result which, although no doubt liable to future modification, 
when a larger comparison shall have been made, proves, nev¬ 
ertheless, that many of the species had a wide geographical 
range. It seems that comparatively few of the gasteropods 
and lamellibranchiate bivalves of l^orth America can be 
identified specifically with European fossils, while no less 
than two-fifths of the brachiopoda, of which my collection 
chiefly consisted, are the same. In explanation of these facts, 
it is suggested that most of the recent brachioj)oda (especial- 
1}^ the orthidiform ones) are inhabitants of deep water, and 
that they may have had a wider geograj^hical range than 
shells living near shore. The predominance of bivalve mol- 
lusca of this peculiar class has caused the Silurian period to 
be sometimes styled “ the age of brachiopods.” 
In Canada, as in the State of New York, the Potsdam 
Sandstone underlies the above-mentioned calcareous rocks, 
but contains a different suite of fossils, as will be hereafter 
explained. In parts of the globe still more remote from Eu¬ 
rope the Silurian strata have also been recognized, as in 
South America, Australia, and India. In all these regions 
the facies of the fauna, or the types of organic life, enable us 
to recognize the contemporaneous origin of the rocks; but 
the fossil species are distinct, showing that the old notion of 
a universal diffusion throughout the ‘‘ primseval seas ” of one 
uniform specific fauna was quite unfounded, geographical 
provinces having evidently existed in the oldest as in the 
most modern times. 
