122 AMERICAN FOSSIL BRYOZOA. [bull. 173. 



extent obliterated, and they can rarely be successfully sectioned for 

 study. Sometimes their substance has been dissolved away, leaving- a 

 perfect mold in the matrix. A gutta-percha impression will then 

 often give a very satisfactory idea of the exterior of the original 

 fossil. 



The best specimens are usually obtained from the shales between or 

 just above or below limestone layers. The smaller forms may be ob- 

 tained free by carefully washing the shales and picking them out from 

 the debris. Some kinds of shales or clay will wash away better if first 

 allowed to become thoroughly dry. Others do better if allowed to 

 soak in water for a longer or a shorter period of time. 



Often the surface characters are obscured by the clayey matrix. 

 This may be removed by the use of caustic potash. The deliquescence 

 of small pieces of this substance, which needs to be handled gingerly 

 with unprotected hands, laid upon the fossil loosens the clay, which is 

 then easily brushed off. Some workers accomplish the same result by 

 placing their specimens in a saturated solution of Glauber's salts, which 

 in crystallizing loosens the clay. 



MAKING OF SECTIONS. 



The preparation of thin sections for microscopic examination is 

 indispensable if one would understand the bryozoa. Directions for 

 making sections of fossil bryozoa, more particularly the Paleozoic, are 

 given in the report of the Geological Surve}" of Illinois, VIII, 1890, 

 p. 292, and in the Geology of Minnesota, III, 1893, p. 100, and with 

 a few additional notes are here repeated. Some experience and con- 

 siderable care are required to produce satisfactory sections. In the 

 absence of a lathe or machine for cutting rock sections, the following 

 method will give as good and, with experience, even better results than 

 the lathe. 



The materials required are (1) a piece of sandstone, not too gritty, 

 8 or 10 inches wide, 18 or 20 inches long, and of sufficient thickness to 

 insure stability; (2) a water hone 1 inch thick, a little wider, and 4 or 5 

 inches long; (3) a block of wood (walnut is the best) 1 inch thick, 2 

 inches wide, and 4^ inches long. The edges of the upper side of this 

 block should be rounded to fit the hand, while in the lower side a shal- 

 low excavation lyV hy 3^ inches is made to fit the ordinary glass slip. 

 The excavation must be so made that the central portion of the glass 

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



The procedure for sectioning specimens large enough to be handled 

 without difficulty is as follows: With a strong pair of "wire nip- 

 pers" a fragment is pinched from the specimen to be sectioned. 

 This fragment is rubbed upon the sandstone until the surface is per- 

 fectly flat. In doing this the greatest care must be exercised to retain 

 or obtain, as the case may be, the desired angle. This surface is 



