636 BRITISH PLIOCENE STRATA. 



Distrihution of the above Marine Testacea, 



Number of Species. Species common to the 



Norwich Crag - - - - 81 I Norwich and Red Crag (not in Cor.) 33 



Red Crag - - - - 225 I Norwich and Coralline (not in Red) 4 



Coralline Crag - - - 327 | Red and Coralline (not in Norwich) 116 



1 Norwich, Red, and Coralline - 19''^ 



Proportion of Recent to Extinct Species. 



Percentase of 

 Eecent. Extinct. Eecent. 



Norwich Crag - - - - 69 - 12 - 85 



Red Crag - - ... 130 . 95 - 57 



Coralline Crag - - . . 168 - 159 - 51 



Recent Species not living noiu in British Seas, 



Northern Species. Southern. 



Norwich Crag ----12 - 



Red Crag .... 8 - 16 



Coralline Crag - - - 2 - 27 



In the above list I have not conckided 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 ; butf 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 echinatiun. 



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 

 Red 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 

 pygmcea; whereas the Red Crag contains, as stated in the table, 8 

 Northern species, all of which recur iu 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 horealis, 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. 



