210 INCKEASING COLD SHOWN BY CHAP. xn. 



In the above list I have not included 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. The land and fresh 

 water shells, thirty-two in number, have also been purposely 

 omitted, as well as three species of London Clay shells, sus 

 pected 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 differ 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 

 common, 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 twenty-seven southern shells, 

 including twenty-six Mediterranean, and one West Indian 

 species (JErato Maugerice). Of these only thirteen occur in 

 the Eed Crag, associated with three new southern species, 

 while the whole of them disappear from the Norwich beds. 

 On the other hand, the Coralline Crag contains only two arctic 

 shells, Admete viridula and Limopsis pygmcea ; whereas 

 the Eed Crag contains, as stated in the table, eight northern 

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

 addition of four 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 sup 

 posing 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 

 united by their ligament, in company with other littoral shells, 

 such as Mya arenaria and Littorina rudis, and evidently 



