FOSSILISATION. 5 



the domain of the neozoologist or the neobotanist, and would not 

 rightly be termed " fossils." It must be admitted, however, that in 

 approaching the " Recent " period of the earth's history, it becomes 

 a matter of difficulty — in some cases an impossibility — to draw any 

 precise line between fossil and recent specimens. 



The terms " fauna " and " flora " are employed in Palaeontology 

 much as they are by the student of recent forms, to mean the entire 

 assemblage of the animals or of the plants respectively belonging to 

 a particular region or a particular time. Thus we may speak of the 

 " fauna " of the Carboniferous Period, or the " flora " of the Tertiary 

 Epoch, or the fauna of the Chalk, or of any other set of beds. 



Fossilisation. 



The term " fossilisation " may be applied in a general sense to all 

 the processes through which an organic body passes in order to be- 

 come a fossil. Here we need only consider the three leading forms 

 in which fossils present themselves. In the first instance, the fossil 

 is to all intents and purposes an actual organic remain, being itself a 

 fragment of an animal or plant. Thus we may meet with fossil 

 bones, shells, or wood, which may have undergone certain changes, 

 such as would be produced by pressure, by the deprivation of or- 

 ganic matter originally present, or by more or less complete infiltra- 

 tion 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 almost all formations contain 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 

 pressure, also, of the surrounding matter would ensure that the clay 

 would everywhere adhere 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 obviously have the following state of parts. The clay which 

 filled the shell would form an accurate cast or mould 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 from one another, since the inside of a shell is very unlike the 



