326 



PRINCIPLES OF ANIMAL BIOLOGY 



Soft parts have sometimes been preserved merely as films of carbon, 

 which is the residue of the protoplasm. These films outline the body 

 perfectly, around the skeleton which retains more nearly its original 

 condition. 



Entire insects in coniferous forests of the Oligocene epoch became 

 immersed in the sticky resin on the bark of the trees, which then hardened, 

 and may still show the delicate spines or the scales of the wings in butter- 

 flies as clearly as in the original. 



More often only the hard parts are left — the tubes of corals (Fig. 273), 

 the shells of clams, the bones of vertebrate animals. Usually these hard 



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I'lci. 272. — Muniinoth found fruzeu in .Sil)eiia in lUUl. Most of the fle.sh was still on 

 the body and intact. The skin is mounted in the museum of Leningrad in the posture in 

 which it was found. {From Lull, "Organic Evolution," courtesy of The Macmillan Company.) 



parts must be buried before disintegration has proceeded far. They may 

 rest at the bottom of a lake, and be covered by silt carried in from the 

 land; they may lie on flood plains of streams and be buried under deposits 

 at times of high water; they may sink in the soft mud of bogs, be buried 

 in wind-blown dust, or covered with volcanic ash. Very often the 

 burying material hardens into rock by the cementing action of ground 

 water carrying minerals; this is particularly true of under- water deposits. 

 After such hardening, the shape of the buried object is usually main- 

 tained, regardless of what becomes of the material of which it is composed. 

 Sometimes the entire buried shell or bone is dissolved away by ground 

 water, which usually contains some carbonic acid (carbon dioxide in 

 solution). The cavity thus left is a mold. If this cavity is later filled 



