286 OUR COMMON BRITISH FOSSILS. 



up into islands and reefs, among and around which 

 the Red Crag was deposited during a period of slow 

 subsidence. 



Professor Prestwich thinks that the more southern 

 species of mollusca which had migrated thus far north 

 during the later Miocene period, and whilst the 

 Belgian deposits were forming, were replaced in the 



Coralline Crag by an as- 

 semblage partly northern 

 and partly southern. That 

 is to say, we have un- 

 doubtedly proofs of a slow 

 but sure migration of nor- 

 thern forms. This may 

 have resulted from a gene- 

 ral lowering of the tempera- 

 ture, or by the setting in 



Fig. zS-L.Cyprina Islandica. (Crag 



formations and recent). Q f {^^ currents from a 



northerly direction, owing to the subsidence of land 

 in that direction. Among the common northern 

 species of mollusca are Astarte sulcata, Glycimeris 

 siliqua, Tellina calcarea, Buccinopsis Dalei, Emargi- 

 nula crassa. Professor Prestwich is further of opinion 

 that there are different zones in the Coralline Crag, to 

 be distinguished by the occurrence of characteristic 

 shells. Mr. Searles Wood described 322 species of 

 Coralline Crag shells ; whilst five more species of 

 Brachiopoda bring up the number to 327. The late 

 Dr. Woodward based certain calculations on the data 



