246 



ZONES OF DEPTH. 



mum as low down as from 35 to 55 fathoms, and 

 above and below this the specific forms decrease 

 progressively : out of twenty-four species, one-half 

 are dredged between those depths. None live in the 

 marginal zone ; and one [Pleurotoma abyssicola) 

 was found below 100 fathoms. 



In our own seas the Pleurotomce belong mostly 

 to deep-sea zones ; so, also, over the intermediate 

 region of the Lusitanian Atlantic ; out of twenty- 

 three species obtained by Ed. Forbes from the 

 iEgean, one-third, at least, are British. 



The study of the distribution of marine life 

 according to zones of depth, suggests to the pa- 

 laeontologist many useful cautions ; it teaches him 

 that under different depths, and in the distinct de- 

 posits forming there, are assembled characteristic 

 suites of animals, living apart, which when they die 

 are entombed apart, and leave there the evidences 

 of their past existence. These assemblages are as 

 distinct from one another as are those which cha- 

 racterize the subdivisions of the deposits of older 

 times, whether tertiary, secondary, or palaeozoic. 



The sublittoral zone of every sea and ocean pre- 

 sents the fulness of its fauna, and from that it de- 

 creases progressively and rapidly, till in regions far 

 within those over which the finer sedimentary de- 

 posits are distributed, animal life altogether ceases. 

 Far beyond the zones where the members of a marine 

 fauna live, there are areas of wide extent, where ani- 

 mals of oceanic habits strew their delicate structures : 



