v.] INTRODUCTION. Xciii 



our Coralline Crag beds ; and it survives in the North 

 Atlantic under the name of C. Couthouyi. The Cardita 

 senilis of the same beds is the C. sulcata of the Medi- 

 terranean; and the Crag C. scalaris is the C. borealis 

 of Conrad and inhabits the coast of Massachusetts and 

 the Arctic Sea. Many other instances of a similar kind 

 might be given. Some species appear to be more hardy 

 than others and have consequently resisted considerable, 

 and perhaps frequent, changes of temperature or climate. 

 Littoral or shallow- water species are of course the most 

 liable to be exterminated or affected by such changes, 

 and the instances above given are of that kind. Many 

 of the Thibetan and Algerian land-shells belong to 

 European species ; and thus the chain of relationship to 

 which I have referred is complete. 



Our upper Tertiary fossils offer tolerable evidence that 

 the climate of this country was, previously to or at the 

 time of their being deposited, of a Glacial or Arctic 

 character, as will be seen by referring to the table of 

 distribution of our land and freshwater Mollusca at the 

 end of this volume. Nearly all the land-shells which 

 occur in the pleistocene strata, but are not now living 

 in Great Britain {e. g. Helix fruticum, H. incarnata, and 

 H. ruder ata), are decidedly Northern species, inhabiting 

 Finland and Scandinavia ; and even the Alpine variety 

 of H. arbustorum appears to be the only form of that 

 species which has been found in our Tertiaries. Among 

 the freshwater shells in this same formation, Corbicula 

 (or Cyrena) fluminalis presents, however, an apparent 

 but remarkable exception from the above indication of 

 our ancient climate having been so severe, if the habits 

 of that species have not undergone any change. The 

 Corbicula is only known to live at present in Asia. But 

 it may be observed that a common European freshwater 



