12 PRINCIPLES OF PALEONTOLOGY. 



almost precisely in its original condition, and even with its 

 soft parts uninjured. More 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 applied to that 

 organ ; whilst shells 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 interior of the 

 shell, and the clay outside would give us an exact impression 

 or cast of the exterior of the shell (fig. i). We should have, 



then, two casts, an interior and 

 an exterior, and the two would 

 be very different to one another, 

 since the inside of a shell is 

 very unlike the outside. In 

 the case, in fact, of many uni- 

 valve shells, the interior cast or 

 "mould" is so unlike the ex- 

 terior cast, or unlike the shell 



Fig. i. Trigoma longa, showing casts itself, that it may be difficult tO 



Cretecw^N^o^*^ 111 * ^ determine the true origin of the 



former. 



It only remains to add that there is sometimes a further 

 complication. If the rock be very porous and permeable by 



