Lower Silurian Fauna of Minnesota. 327 



aration some changes of plan were made until now the list con- 

 sists only of Lower Silurian species. 



Experience in collecting convinced me that the division into 

 Trenton limestone, Trenton shales and Galena limestone was too 

 indefinite for any practical use in marking the vertical range of 

 fossil remains. The plan, too, of designating the elevation in feet 

 above the top of the Saint Peter formation, was found inconven- 

 ient and misleading. As a consequence I finally decided to lay aside 

 the old terminology used in Minnesota, and describe the beds as I 

 find them; to identify what I could with the beds that are de- 

 scribed in Wisconsin and to give names to such as had to be de- 

 fined anew. The terms used in the preceding paper (p. 325) will 

 reappear in this for the designation of several beds. 



The matter of correct identification was more difficult, but in 

 each case the material identified has been labeled and stored 

 away, so that corrections can be made without disturbing the facts 

 as to distribution. This serves my purpose as well as that of per- 

 sons wishing to make exchanges for Minnesota fossils, it being 

 the aim of the Department of Geology in the University of Minne- 

 sota (for which the material used was collected) to supply such 

 wants when possible. 



To some species no descriptions were found at all and conse- 

 quently they had to be described before exchange with them was 

 practicable. 



I have made a number of sketches to accompany descriptions 

 of fossil forms that for reason of being new, or at least supposed 

 to be new, have been thought worth describing. 



The described forms may be considered either as species or 

 varieties without affecting in the least the purpose of this article. 

 That they are distinct forms I deem sufficient for my present pur- 

 pose, and without any attempt at a detailed discussion upon their 

 classification, the descriptions are offered. 



In the preparation of the table below, no species has been in- 

 cluded that is not in the collections belonging to the Department ; 

 and distribution is indicated only where these species have been 

 found. Question marks indicate doubt as to identification and 

 not as to the occurrence of the species in hand. 



