. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palaeontological science. Paleontology. 12 PRIXCIPLES OF PALAEONTOLOGY. almost precisely in its original condition, and even with its soft parts uninjured. ^lore commonly, certain changes have taken place in the fossil, the principal being the more or less total removal of the organic matter originally present. Thus bones become light and porous by the removal of their gela- tine, so as to cleave to the tongue on being apphed to that organ; whilst sheUs become fragile, and lose their primiti
. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palaeontological science. Paleontology. 12 PRIXCIPLES OF PALAEONTOLOGY. almost precisely in its original condition, and even with its soft parts uninjured. ^lore commonly, certain changes have taken place in the fossil, the principal being the more or less total removal of the organic matter originally present. Thus bones become light and porous by the removal of their gela- tine, so as to cleave to the tongue on being apphed to that organ; whilst sheUs become fragile, and lose their primitive colours. In other cases, though practically the real body it represents, all the cavities of the fossil, down to its minutest recesses, may have become infiltrated with mineral matter. It need hardly be added, that it is in the more modern rocks that we find the fossils, as a rule, least changed from their former condition; but the original structure is often more or less com- pletely retained in some of the fossils from even the most ancient formations. In the second place, we very frequently meet with fossils in the state of " casts" or moulds of the original organic body. What occurs in this case will be readily understood if we ima- gine any common bivalve shell, as an Oyster, or Mussel, or Cockle, embedded in clay or mud. If the clay were sufficiently soft and fluid, the first thing would be that it would gain access to the interior of the shell, and would completely fill up the space between the valves. The pressure, also, of the surround- ing matter would insure that the clay would everywhere ad- here closely to the exterior of the shell. If now we suppose the clay to be in any way hardened so as to be converted into stone, and if we were to break up the stone, we should obvi- ously have the following state of parts. The clay which filled the shell would form an accurate cast of the interio?' of the shell, and the clay outside would give us an exact impressi
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