636 BRITISH PLIOCENE STRATA. 



Distribution of the above Marine Testacea. 

 Number of Species. Species common to the 



Norwich Crag 81 



Red Crag ----- 225 



Coralline Crag - - - 327 



Norwich and Red Crag (not in Cor.) 33 

 Norwich and Coralline (not in Red) 4 

 Red and Coralline (not in Norwich) 116 

 Norwich, Red, and Coralline - 19° 



Proportion of Recent to Extinct Species. 



Percentage of 

 Eecent Extinct. Eecent, 



Norwich Crag - 69 - 12 - 85 



Red Crag 130 - 95 - 57 



Coralline Crag ... 168 - 159 - 51 



Recent Species not living note in British Seas. 



Northern Species. Southern. 



Norwich Crag - 12 - 



Red Crag - 8 - 16 



Coralline Crag - - - 2 - 27 



In the above list I have not concluded the shells of the glacial beds 

 of the Clyde and of several other British deposits of newer origin than 

 the Norwich Crag, in which nearly all — perhaps all — the species are 

 recent, although such fossils are described by Mr. Wood, or enumer- 

 ated in his Appendix. The land and freshwater shells, 32 in number, 

 have also been purposely omitted, as well as three species of London 

 Clay shells, suspected by Mr. Wood himself to be spurious. 



By far the greater number of the recent marine species included in 

 these tables are still inhabitants of the British seas ; but even these dif- 

 fer considerably in their relative abundance, some of the commonest of 

 the Crag shells being now extremely scarce ; as, for example, Buccinum 

 Dalei, and others, rarely met with in a fossil state, being now very com- 

 mon, as Murex erinaceus and Cardium echinatum. 



The last table throws light on a marked alteration in the climate of 

 the three successive periods. It will be seen that in the Coralline Crag, 

 there are 2*7 Southern shells, including 26 Mediterranean, and one 

 West Indian species [Erato Maugerce). Of these only 13 occur in the 

 Bed Crag, associated with 3 new Southern species, while the whole of 

 them disappear from the Norwich beds. On the other hand, the Coral- 

 line Crag contains only 2 Arctic shells, Admete viridula and Limopsis 

 pygmma; whereas the Bed Crag contains, as stated in the table, 8 

 Northern species, all of which recur in the Norwich Crag, with the 

 addition of 4 others, also inhabitants of the Arctic regions; so that 

 there is good evidence of a continual refrigeration of climate during 

 the Pliocene period in Britain; The presence of these Northern shells 

 cannot be explained away by supposing that they were inhabitants of 

 the deep parts of the Sea ; for some of them, such as Tellina calcarea 

 and Astarte borealis, occur plentifully, and sometimes with the valves 



* These 19 species must be added to the numbers 33-4 and 116 respectively, 

 in order to obtain the full amount of common species in each of those cases. 



