MATERIALS 43 



are impressions of the outer surface, and " moulds " 

 when they consist of matrix that filled the internal 

 cavity. Clear casts are hardly inferior to preserved 

 shells for purposes of study, since wax-squeezes will 

 restore to positive proportions all the surface characters 

 of the dissolved fossils ; while moulds are, in many cases, 

 far more instructive objects than perfect shells. It is 

 exceedingly difficult to dissect out the hinges or muscle- 

 impressions of Pelecypoda or Brachiopoda set in re- 

 fractory matrix, but the full structure of these internal 

 features can be examined on moulds, or on artificial 

 positives taken therefrom. Indeed, so useful are natural 

 moulds in this respect, that methods have been devised 

 for the artificial removal of preserved shells, with intent 

 to reveal internal structures impressed on the infilling 

 matrix. Fig. 2, PI. iii., and fig. 6, PI. iv., show how 

 admirably the features of fossils may be represented by 

 this mode of " destructive preservation." 



The fine sandstones of the Shropshire Ordovician 

 contain " rotten-bands " caused by innumerable casts and 

 moulds (PI. vi. fig. 2), while the Portland Oolite is a 

 well-known example of a similarly carious limestone. 

 As a general rule, this style of preservation represents 

 all kinds of fossils in arenaceous or cherty rocks, while 

 in calcareous matrices fossils of aragonite and opaline 

 silica are usually indicated by negatives. 



Under conditions of slow or intermittent deposition, 

 empty shells may become filled with concretionary 

 matter differing from the matrix by which they are 

 subsequently surrounded. The shells themselves are 

 often removed before burial is completed, so that 

 moulds, without casts, are the sole relics of the organ- 

 isms. Glauconite is often found in grains representing 

 cavities of Foraminiferal chambers, while impure phos- 

 phate of lime (" coprolite ") similarly occurs as con- 



