[27] COLLECTING AND PREPARING FOSSILS SCHUCHERT. 



many changes of water to wliich lias been added a few drops of muri- 

 atic acid, and brush repeatedly. Sometimes the white iilm is a product 

 of decomposition and can not be wholly removed. It can then be dark- 

 ened with India ink or some suitable color. 



Preparing sJceletons. — The cleaning of vertebrate skeletons when em- 

 bedded in rock is at best very difficult work. Since none but large 

 institutions attempt to prepare such bones, there will be no necessity 

 to describe here the various manipulations such specimens undergo 

 before they are placed on exhibition. However, all bones found in 

 marshes should at once be well soaked in gum water after exhuming, 

 and then dried slowly in a i^rotected i)lace. Bones found loose on the 

 surface should also be treated in the same way. Boiling in oil, shel- 

 lacking, or varnishing should be avoided. 



ARTIFICIAL CASTS AND SQUEEZES FROM NATURAL MOLDS. 



When the specimen is large and flat, as large Cambrian trilobites, for 

 instance, dental plaster of paris will m.ake good casts. If the specimen 

 is in a hard slate or limestone, moisten it well with water or apply a 

 thin solution of "green soap" before pouring on the liquid plaster of 

 paris, which is to prevent the cast from adhering to the fossil. 



To obtain a squeeze of the dentition of shells or delicate surface 

 ornamentation from not too deep natural molds, gutta-percha is very 

 good. This comes in small, thin sheets of a reddish color. Out off a 

 piece about twice the size of the mold. Heat this in hot water and 

 then loress it into the moistened mold with the thumb, which must.be 

 wet to prevent sticking. Considerable pressure is required to make a 

 fine squeeze. Keep folding back into the middle that squeezed out, 

 otherwise there will not be enough gutta-percha in the deep parts to 

 take on the form of the mold. 



Where the mold is very deep or has slight "undercuts," a very good 

 substance to use for making casts is "modeling composition fo^ dental 

 purposes, No. 2, medium." This comes in cakes packed in half-pound 

 boxes, and can be purchased at any dental supply store. This also is 

 made pliable by hot water. Modeling composition is used for making 

 squeezes in the same manner as the former, and since it is softer when 

 heated and remains so longer than gutta-percha, it can be more readily 

 pressed into deep cavities and bent out of undercut places. 



Casts of gutta-percha and modeling composition in the course of 

 years become very brittle and are then very easily broken. If perma- 

 nency is required, molds and casts in plaster of paris should be made. 



METHODS OF PREVENTING DECOMPOSITION. 



About the only fossils which are liable to decompose in the air are 

 those changed into iron pyrite. As yet no process is known which 

 will entirely stoj) decomposition and leave the specimen with its natu- 

 ral color. Boiling in hot paraffin or melting this over the fossil with 



