100 THE PALEONTOLOGY OF MINNESOTA. 
{Introduction. 
caleareous 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 Delafield 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. 
Silicified Bryozoa are comparatively of rare occurrence, especially in Lower and 
Upper Silurian rocks. In nearly all cases this method of preservation is contined 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 Canada, and the 
St. Louis limestone of Kentucky; or itis 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. Im 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 Cryptostomata. 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, (1)a piece of sandstone (not too gritty*) eight or ten 
inches wide, eighteen or twenty inches long, and of sufficient thickness to insure 
*The Buena Vista freestone of the Ohio Waverly is the best known to me for the purpose; 
