292 PALEONTOLOGY OF ILLINOIS. 



CHAPTER I. 



GENERAL AND COMPARATIVE STRUCTURE OF PALEO- 

 ZOIC BRYOZOA. 



To get a correct idea of the structure of Palaeozoic Bryozoa, 

 it is absolutely necessary that they be studied with the aid of 

 thin sections. 



The following is a simple method for preparing 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 solidity; (2) a piece of water 

 hone one inch thick, a little wider, and four or five inches long; 

 (3) a block of wood (walnut is the best) one inch thick, two 

 inches wide and four and one-half inches long. The edges of the 

 upper side is rounded to fit the hand, while in the lower side a 

 shallow excavation one and one-sixteenth inches by three and 

 one-eighth is made to fit the ordinary glass slip. The excava- 

 tion must be made so that the central portion of the glass slip 

 will bear upon the block, while the ends may have a little play. 



With a strong pair of "wire nippers" a fragment is pinched 

 from the specimen of which sections are desired. This is taken 

 into the fingers, rubbed upon the sandstone until the surface is 

 perfectly flat. This is the most important part of the process, 

 and the greatest care must be exercised to retain (or obtain, as 

 the case may be,) the desired angle. This surface is now rubbed 

 smooth upon the hone, when the fragment is ready for mount- 

 ing. A drop of Canada balsam is placed upon the glass slip 

 and the ground face of the fragment into it. The slip is now 

 heated (on a heating stage or over a lamp) and the Canada 

 balsam allowed to boil five or six seconds, when the slip is laid 

 upon a horizontal piece of wood to cool. After it is cold the 



