CHAPTER lY. 



ON LOWER SILURIAN BRYOZOA OF 



MINNESOTA. 



BY E. O. ULKICH. 



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 plentiful 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 develop- 

 ment. In short, of every impartial collection of the Lower Silurian fossils ot Minne- 

 sota, the Bryozoa necessarily constitute a large portion, not only of the number of 

 species and specimens, 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 geo- 

 graphical 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 cor- 

 relations of strata at widely separated localities cannot be overestimated. Many of 

 them, especially of the suborder Trepostomata, are serviceable even where other fossils 

 are too imperfect, since with the aid of thin sections mere fragments can often be 

 identified with certainty. 



