GEAR THE Ey: 
ON LOW ER SLU RIAN. BRMOZO 22 On 
MINNESOTA. 
BY E, 0. ULRICH. 
INTRODUCTORY REMARKS. 
To the Bryozoa must be accorded the first rank among the various classes of fossils 
that are represented in the Lower Silurian rocks of Minnesota, They are entitled to 
this distinction, first, because of the great variety of form and structure found among 
them, and, second, because of their exceeding abundance, in the way of individuals. 
In both of these respects their representation exceeds that of the Brachiopoda, which 
doubtlessly held the second rank, in the approximate ratio of two to one. So plen- 
tiful are their remains in some of the beds, particularly in the shaly members, that 
they may be said to constitute no inconsiderable part of the strata. In the Trenton 
shales the intercalated plates of limestone are literally covered with them, and they 
are not rare even in the massive limestones above and beneath the shales, which 
were deposited under conditions much less favorable to their development. In short, 
of every impartial collection of the Lower Silurian fossils of Minnesota, the Bryozoa 
necessarily constitute a large proportion, not only of the number of species and spe- 
cimens, but of its bulk as well. 
The importance of the Bryozoa from the view of the stratigraphical geologist, is 
again second to no other class of fossil remains. Many of them have a wide geograph- 
ical distribution, and as they usually occur in greater or less abundance, and are very 
persistent in their characters, their value as data upon which to base correlations of 
strata at widely separated localities cannot be overestimated. Many of them, espe- 
cially of the suborder J'repostomata, are serviceable even where other fossils are too 
imperfect, since with the aid of thin sections mere fragments can often be identified 
with certainty. 
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