The Principles of Palaeontology. 213 



The first group contains matter of purely organic origin, 

 as chitine and conchioline. These substances are eventually 

 attacked by disintegrating agents, but they resist these long 

 enough before they disappear to leave hollow casts, or else they 

 change into carbonaceous deposits which faithfully reproduce 

 their forms. 



The localities in Bavaria already cited, contain Crustacea, 

 Arachnids and Insects in excellent state of preservation. These 

 last named are found also by thousands in the lacustrine de- 

 posits of Aix, Armissan, dating from the Oligocene, of Oeningen 

 (Baden), of Florissant (Colorado), (Miocene). Among the impres- 

 sions of chitinous substances we must cite the Graptolites, 

 hydrozooid colonies which abound in the feilurian beds of 

 Bohemia, Sweden and America. 



Shells, the covering of Echinoderms, and the bones of Verte- 

 brates are composed of calcareous salts mixed with an organic sub- 

 stance, conchioline or ossine. The mineral part consists of 

 carbcnate of lime in the state of arragonite or calcite, or of 

 mixed carbonate and phosphate. The organic substance, as a 

 general rule, decomposes quickly after the death of the animal. 

 The remains found, whether shells or bones, then sometimes 

 remain porous. But usually water charged with calcareous salts 

 penetrates into the interstices thus produced and the salts 

 (carbonates or phosphates) are deposited in such a manner that 

 the remains become homogeneous. 



Yery frequently, during deposition on the bottom where the 

 organic remains lie, the sediment penetrates into the cavities 

 which remain between the hard parts; the interstices in the 

 skeleton of the corals, the visceral cavities of the Sea-urchins, the 

 shell cavities of Mollusks and of Brachiopods are thus frequently 

 filled with a substance identical with the contiguous rock ; 

 this may be carbonate of lime, clay, sand, more rarely flint, oxide 

 of iron or phosphate of lime. The original shell may also be 

 preserved, but it often disappears, being dissolved by water 

 charged with carbonic acid. In this case the fossil pre- 

 sents itself in the form of an internal mold. If the cover- 

 ing is thin, the examination of the mold may sufiice to give an 

 idea of the shell itself ; this is the case notably among the 

 Ammonites, which are generally found in this state; but it often 

 happens, on the other hand, that the interior of a fossil cavity 

 gives only a vague idea of the details of the exterior, and a 

 decision regarding its external features presents great difficulties. 

 Such is the case with many Acephala and Gasteropoda. 



A fossil naturally leaves an impression on the rock which con- 

 tains it ; this impression representis the external mould of the fos- 

 sil. There is often an internal mould together with the external 

 one ; in this case the original form of the fossil itself can often be 

 artificially reproduced. In order to do this the space left vacant 

 must be filled with soft plaster or wax, and the rock then can be 



