TECHNIQUE 63 



or even family status is not apparent The principles 

 whereby such groups as Corals, Brachiopods and 

 Pelecypods are classified are based largely upon features 

 invisible on external examination. It thus becomes 

 necessary to excavate the included matrix, with more or 

 less inevitable destruction of the surface of the fossil 

 examined. Fortunately, imperfect specimens are more 

 abundant than complete ones, so that the wastage con- 

 sequent on dissection can usually be limited to examples 

 unsuited to fill honoured places in a collection. 



The mode of dissection adopted will be determined 

 partly by the condition of the matrix, and partly by 

 the nature of the organism. There are three methods 

 generally available for exposure of internal structures in 

 fossils. If the infilling is loose, a portion of the shell 

 may be filed away, and the interior cleared by means 

 similar to those used for the outer surface. If the con- 

 tained matter is hard (a frequent condition, especially in 

 cases where concretions develop), the shell may be 

 entirely removed, leaving a mould which shows internal 

 features in negative. If neither of these methods is 

 satisfactory, resort may be had to sectioning fossil and 

 matrix together. 



For the study of Corals and similar forms, the last- 

 named process is necessary. Transverse and longi- 

 tudinal sections can be obtained by grinding specimens 

 to the required level, and either polishing the surface or 

 preparing a thin slice for transparency. By cutting the 

 specimen transversely, a series of sections can be taken 

 across a single corallite at different levels, to display 

 developmental stages. In many cases, where a fossil is 

 too delicate for extraction, or encased in very refractory 

 matrix, serial sections comparable with those made by a 

 microtome can be prepared. The sections cannot be pre- 

 served like those in a " ribbon " of paraffin wax; each is 



