CHAPTER IV. 



ON LOWER SILURIAN BRYOZOA OF 



MINNESOTA. 



BY E. O. ULEICH. 



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 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. 



[96] 



