6 OUR COMMON BRITISH FOSSILS. 



doing my best to introduce the young student to the 

 "happy hunting-grounds" of our various geological 

 formations. I intend to limit myself to the common 

 fossils, unless occasionally tempted to mention a few 

 for the purpose of further whetting the appetite. 

 And, whilst I describe the spots where the young 

 geologist is most likely to " make a bag," I shall give 

 a brief description of the natural history relationships 

 of the numerous extinct organisms. 



For a long time it was the practice to regard the 

 most doubtful fossil marine organisms, which had no 

 particular shape or external structure by which they 

 could be at once recognized, as " a kind of sponge." 

 Thanks, however, to the labours of such men as 

 Hinde, Sollas, and Carter, it is no longer possible to 

 avail ourselves of such a fossil lumber-room, where 

 our ignorance and mistakes can be screened. For, 

 although there is some doubt and disputation as to 

 the exact zoological rank of sponges whether they 

 belong to the Protozoa or Ccelenterata there is little 

 concerning their habits and structure. The extended 

 use of the microscope in geology, and the practice of 

 cutting transparent sections of any doubtful fossil, so 

 as to enable the student to identify it by its organi- 

 zation, at once enables him to detect " sponge 

 structure," or rather the mineral part of that structure. 

 For in fossils the sponge-flesh, or sarcode as it is 

 called, has of course disappeared, and left no trace 

 behind it. 



