TECHNIQUE 65 



the specimen may be heated (insufficiently to induce 

 chemical change) and suddenly plunged into cold water. 

 The laminate fabric of the shell tends to flake off, and 

 careful repetition of the process will expose a clean 

 mould. Recent application of this method to Jurassic 

 Brachiopoda has almost revolutionized the classification 

 of some groups. 



(IV) STAINING 



In -those fossils that have sutural lines or fine orna- 

 ment and pores, it is often difficult to detect details 

 owing to the similarity in colour between shell and 

 matrix. This applies particularly to fossils from the 

 Chalk. In those, the porosity of the rock and relative 

 compactness of the fossil invite application of a soluble 

 stain. This is absorbed by the chalk, and may be 

 washed off from the calcite. By such means, ambulacral 

 pores or Polyzoan zooecia are clearly brought out. The 

 tracing of sutures is important in the study of Echino- 

 dermata, Cephalopoda and Trilobita. In Crinoids and 

 Echinoids, divisions between the plates can be detected, 

 or made more apparent, by suitable treatment. On well- 

 preserved surfaces, suture-lines occur as very fine grooves. 

 These can be intensified by application of a stain giving 

 a granular precipitate. One of the best reagents is 

 Indian Ink freshly diluted with water. A few moments 

 after the fluid has been painted on to the moistened 

 surface, before it has begun to dry, the whole specimen 

 should be lightly rubbed with the finger or a cloth. The 

 stain on smooth parts is removed, while that in depres- 

 sions is left. A brush must not be used, since the bristles 

 scour out the grooves and give disconcerting results. 

 An excellent method for displaying suture-lines on 

 pyritous moulds of Ammonites consists in soaking the 

 specimen in dilute size, and rubbing chalk, or prefer- 

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