THE SCOPE OF PALEONTOLOGY. 13 



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.Triffonia longa, showing casts itself, that it may be difficult to 



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 

 water, it may happen that the original shell is entirely dissolved 

 away, leaving the interior cast loose, like the kernel of a nut, 

 within the case formed by the exterior cast. Or it may happen 

 that subsequent to the attainment of this state of things, the 

 space thus left vacant between the interior and exterior cast 

 the space, that is, formerly occupied by the shell itself may be 

 filled up by some foreign mineral deposited there by the infil- 

 tration of water. In this last case the splitting open of the 

 rock would reveal an interior cast, an exterior cast, and finally 

 a body which would have the exact form of the original shell, 

 but which would be really a much later formation, and which 

 would not exhibit under the microscope the minute structure of 

 shell. 



In the third class of cases we have fossils which present 

 with the greatest accuracy the external form, and even some- 

 times the internal minute structure, of the original organic body, 

 but which, nevertheless, are not themselves truly organic, but 

 have been formed by a " replacement " of the particles of the 

 primitive organism by some material substance. The most ele- 

 gant example of this is afforded by fossil wood which has been 

 "silicified" or converted into flint (silex}. ^n such cases we 

 have fossil wood which presents the rings of growth and fibrous 

 structure of recent wood, and which under the microscope 

 exhibits the minutest vessels which characterize ligneous tissue, 

 together with the even more minute markings of the vessels 



