610 General and Applied Biology 



In natural molds or incrustations (Fio;, 317) neither the minute struc- 

 tures nor the materials of the original organism are preserved but merely 

 the general outlines of form and shape are recorded. Animals or plants 

 may be enclosed by incrustations of calcium carbonate or silica which 

 harden around the buried organism before it decays. The organic ma- 

 terials of the former organism eventually are removed by decay and a 

 percolation of dissolving waters. The cavity which eventually remains 

 retains the general form and shape of the original organism. In some 



Fig. 319. — Passenger piegon {Ectopistes migratorius) which was once extremely 

 abundant but is now extinct. (Copyright by General Biological Supply House, 

 Inc., Chicago.) 



instances the skeleton has disappeared entirely, leaving only the mold of 

 it as a record. The shells of certain mollusks may have been covered 

 with sediment while the soft parts decayed. The interior then may have 

 been filled with the same sediment. Acidified waters then may have dis- 

 solved the limy shell, leaving only the molds of the exterior and interior. 

 Sometimes the shell is removed, and the space left between the external 

 and internal molds is filled with mineral matters carried in by percolat- 



