PALEONTOLOGY. 133 



While the internal cast thus exhibits in relief all the 

 markings and irregularities which existed on the inner 

 surface of the shell or test, the external cast or impres- 

 sion is indented by all the markings which ornamented 

 its outer surface. Where the shell is absent, therefore, 

 and we wish to gain .an idea of its appearance in relief, 

 we can effect this by taking an artificial cast from the 

 external impression, and thus obtain an accurate repre- 

 sentation of the pre-existing shell. Occasionally even, 

 but very rarely, this is effected by natural means, and 

 we then get an internal cast, showing all the characters 

 of the external shell ; this seems to be only produced 

 in soft sandy marls or muds, where a thin shell having 

 been dissolved and never replaced, the impression of its 

 exterior has been gradually and equally squeezed on to 

 the still soft mud inside ; this, on the desiccation and 

 consolidation of the rock, would present every appear- 

 ance of the original fossil, though the closest examina- 

 tion would reveal no traces of shell structure; in- 

 stances are found in some fossils from the chalk marl 

 and from the Bracklesham beds in the Isle 



L I B R A li V 



CHAPTER If. UNIVE liSITy <; 



PALAEONTOLOGY (continued). 

 Method of Collecting Fossils EiailipleV * f ^ - N 1 A 



I. Method of Collecting Fossils. In Part II. p. 53, 

 the mode of procedure in examining any natural or 

 artificial section, whether cliff, cutting, pit, or quarry, 

 was described, and it was pointed out that notes should 

 be taken of the nature and contents of the rocks therein 



