8i6 NORTH AMERICAN INDEX FOSSILS. 



lest the heat of the gutta-percha soften the glue and cause adhesion. 

 If, however, the destruction of the matrix is not of moment, the 

 gutta-percha may be withdrawn at will, and the adhering dirt 

 soaked and washed ofif at leisure. "^^ 



Temporary casts or squeezes are quickly made in artist's modeling 

 clay or "plastolin" obtainable at any artist's supply store. The 

 squeezes obtained in this material are not so clean and sharp as 

 in the best dental wax, and they are soft and easily destroyed. But 

 the material needs no softening, is always ready for use, and can 

 be used again an indefinite number of times. Besides it is cheap, 

 and so saves cost of making squeezes from large molds. The mold 

 should always be thoroughly moistened, otherwise the plastolin will 

 adhere. Small holes or fissures are also apt to be filled by this 

 material and should be stopped up first. Gutta-percha casts and 

 casts in dental wax become brittle with age. Permanent casts are 

 best made in plaster of paris. 



5. Preparation of Thin Sections. 

 Thin sections are necessary in the study of certain fossils, such 

 as the Bryozoa and the Stromatoporoidea. For purposes of de- 

 velopmental study sections are also important. A revolving plate 

 is useful but not necessary, except for grinding down silicified 

 specimens. These, however, seldom preserve the finer structure. 

 Calcareous fossils may be rubbed down on a hone-stone, or on a 

 glass plate with emery powder and water. The emery should be 

 fine especially for the later rubbing. At first emery of grade 80 to 

 100 may be used, but when the section approaches the point desired, 

 grade 120 to 150 should be substituted, all traces of the coarser 

 grade being first washed off. The final rubbing should be done 

 with pumice powder (obtainable at any druggist's), and water, 

 on a separate glass plate; "jeweller's rouge" or the finest emery 

 dust may likewise be used. Use sufficient water to prevent caking, 

 but not too much. In all cases the specimen must be held very 

 steady and pressed close to the plate, so that the surface produced 

 is a perfectly flat one. Small specimens which cannot be held 

 between the finger and thumb without touching the plate, should 

 be cemented to a square of heavy glass. Specimens ground down 



"Fourteenth Ann. Rept. N. Y. State Geol., 1894, p. 100, footnote. 



