CVlll 



INTRODUCTION. [cH. 



Abundance of Molluscan life. — The whole surface of 

 our globe teems Avith a mass of animal and vegetable 

 life, to which the MoUusca contribute by no means an 

 inconsiderable quota. Owing to the solid and perma- 

 nent nature of their shells, many fossiliferous strata 

 are almost entirely composed of such exuviae ; and this 

 process of accumulation is still going on in the exist- 

 ing sea-bed to an enormous extent. No one can have 

 had any experience in exploring the bottom of our 

 own seas, and examining our tertiary strata, ^vith- 

 out noticing how closely the contents of a well-filled 

 dredge, taken from a submarine shell-bank, resemble 

 the same quantity of material dug out of a crag-pit ; 

 and perhaps nothing can give a more striking idea of 

 the incalculable lapse of time which must have taken 

 place in the history of the world, than the formation of 

 these strata which, after all, are only a few pages of 

 the great book. We here see layer upon layer of organic 

 remains heaped up and compressed, to a depth of thii^ty 

 feet, each layer being only a few inches deep, but repre- 

 senting numerous and successive generations that have 

 long passed away. 



It has not yet been ascertained to what depths mol- 

 luscan life extends. The late Sir James Clark Ross, 

 in the interesting account of his Antarctic Voyage (vol. i. 

 p. 202), says, " I have no doubt that, from however 

 great a depth we may be enabled to bring up the mud 

 and stones of the bed of the ocean, we shall find them 

 teeming with animal life ; the extreme pressure at the 

 greatest depth does not appear to affect these creatures. 

 Hitherto we have not been able to determine this point 

 beyond a thousand fathoms ; but from that depth shell- 

 fish have been brought up with the mud.^^ Still greater 

 depths have been lately reached in recovering the Me- 



