THE ECHINODERMATA 241 



Solid crystalline calcite would be much too heavy a 

 material for an organic skeleton. Accordingly, in the 

 living state the plates are lightened by rounded cavities, 

 arranged so closely as to make the whole substance 

 spongy, and in a very definite organic pattern. This 

 network of calcite is called the stereom. When the 

 skeleton of a recent echinoderm say, the radiole of a sea- 

 urchin is broken, the calcite cleavage is interrupted by 

 these abundant cavities, and is easily overlooked, though 

 careful observation will always show it. But the first 

 process in fossilization is always the filling-in of these 

 cavities with calcite in crystalline continuity with that 

 around them. The pattern of the cavities may or may 

 not be obliterated in the process ; when not, it forms 

 a further means of recognition (besides the crystalline 

 character and cleavage) of echinoderm skeletons in 

 microscopic sections of rocks. Calcite in crystalline con- 

 tinuity may also be deposited around the plates (or other 

 units), if there is opportunity, concealing the organic 

 form and replacing it, as far as circumstances permit, by 

 the geometrical form of a crystal. Thus, on breaking 

 open one of the sea-urchins so common in the Upper 

 Chalk, one occasionally finds that each plate projects 

 into the central cavity as a rhombohedron ; externally 

 the contact of the chalk has prevented such growth, and 

 if the interior has been filled with chalk (the more usual 

 case) the plates will preserve their original surface in- 

 ternally also. 



There is another very general, but not universal, 

 feature characteristic of echinoderms a five-rayed or 



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