2 PRINCIPLES OF PALASONTOLOGY. 
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 zwfertor of the 
shell, and the clay outside would give us an exact impression 
or cast of the exterior of the shell (fig. 1). 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. 1.—Trigonia longa, showing casts itself, that it may be difficult to 
ae aon of the shell— 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 
