4 INTRODUCTION. 



organic remain, being itself a fragment of an animal or plant. 

 Thus we may meet with fcJssil bones, shells, or wood, which 

 may have undergone certain changes, such as would be pro- 

 duced by pressure, by the deprivation of organic matter origi- 

 nally present, or by more or less complete infiltration with 

 mineral matter, but which, nevertheless, are practically the 

 real bodies they represent. As a matter of course, it is in 

 the more modern formations that we find fossils least changed 

 from their primitive condition, but all formations almost con- 

 tain some fossils in which the original structure is more or 

 less completely retained. 



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 imagine 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 pres- 

 sure, also, of the surrounding matter would insure that the 

 clay would everywhere adhere closely to the exterior of the 

 shell. If now we suppose the clay to be in any way hard- 



ened so as to be converted into 

 stone, and if we were to break 

 'y up the stone, we should obvi- 

 ously have the following state 

 of parts. The clay which filled 

 the shell would form an accu- 

 rate cast of the interior of the 

 shell, and the clay outside would 



Fig. l.Trigonia longa, showing casts give US ail exact impression Or 

 of the exterior and interior of the shell ? ,1 , n ,1 i vi 



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 univalve 

 shells, the interior cast is so unlike the exterior or unlike the 

 shell itself, that it may be difficult to determine the true 

 origin of the former. 



