100 THE PALEONTOLOGY OF MINNESOTA. 



[Introduction. 



calcareous shales and limestones. When the shales are of a greenish color, as in parts 

 of the middle third of the Minnesota Trenton shales at Minneapolis, and the shales 

 of the Cincinnati group at Iron Ridge and Delatield in Wisconsin, the internal struc- 

 ture was generally completely destroyed through the coarseness of the crystallization. 

 The same is true in a great measure of forms occurring in dolomitic limestones. 



Silicifted Bryozoa are comparatively of rare occurrence, especially in Lower and 

 Upper Silurian rocks. In nearly all casesthis method of preservation is confined to 

 massive limestones, like the Corniferous and St. Louis, and in most cases it is unfa- 

 vorable, so far as the minute internal structure is concerned. Still, in specimens so 

 preserved, the external characters are often wonderfully perfect. Such specimens 

 have been found at the Falls of the Ohio, where they occurred in the decomposed 

 cherty limestones, from which they were washed free in as perfect a condition, so far 

 as outer features are concerned, as when they were entombed. Silicified specimens 

 may also be freed from the rock by means of dilute acids. 



A rather common condition of preservation in Devonian and Carboniferous 

 deposits, is where the calcareous zoaria have been dissolved away, leaving more or 

 less perfect moulds in the matrix. This is usually a porous chert, like that fre- 

 quently met with in the Corniferous limestone of New York and Cana,da, and the 

 St. Louis limestone of Kentucky ; or it is an arenaceous shale. This method of 

 preservation is often very favorable, since, by pressing heated gutta percha into the 

 empty moulds, it is possible to obtain very serviceable counterparts of the bryozoan 

 that left them. Such casts, if carefully prepared, often bring out the most minute 

 details of external marking with surprising fidelity. In the case of such delicate 

 Bryozoa like the Fenestellidw, these moulds are to be preferred to the usual preservation 

 of calcareous specimens, the latter being too liable to attrition and decomposition. 



METHODS OF STUDY. 



The bulk of paleozoic Bryozoa, with which the American student is likely to be 

 chiefly engaged, belong to the Trepostomata and Cryptostomatu. In these the inter- 

 nal structure is of very diverse types, and it is impossible to arrive at a clear concep- 

 tion of them without the aid of thin sections. If possible, these should be prepared 

 by the student himself, and even if he cannot command one of the new slicing 

 machines, he may still obtain very excellent results by the simple home-made method 

 which I am about to describe, and which served me in making thousands of sections. 



The materials required are, (l)a piece of sandstone (not too gritty*) eight or ten 

 inches wide, eighteen or twenty inches long, and of sufficient thickness to insure 



Tlie BIIIMIU Vista freestone of the Ohio Wuverly is the best known to me for the purpose. 



