VI.] INTRODUCTION. Cvii 



siderably ; but its vertical range may be stated as ex- 

 tending from 10 to 50 fathoms. As a general rule, rocks 

 do not occur in this zone, especially in the deeper part 

 of it stones, gravel and sand (sometimes mixed with 

 mud) being its chief characteristics. These mineral 

 conditions to some extent regulate the nature of the 

 Mollusca which are here found. The whelk-tribe, as 

 well as many Nudibranchs, frequent the stony or, as it 

 is called, " hard " ground ; and different sorts of bivalves 

 take up their quarters in the other, or " soft," ground. 

 Seaweeds are scarce in this zone and are generally 

 absent from its lower regions ; so that most of the Mol- 

 lusca which inhabit it are animal-eaters, some being 

 sarcophagous, others zoophagous, and many of them 

 preying on each other. 



The fourth and last is called the DEEP-SEA zone, and 

 reaches from 50 fathoms to the greatest depth comprised 

 within the line of soundings. Both this and the last- 

 mentioned zone contain our most productive fishing- 

 banks ; and the floor of these submarine areas is exceed- 

 ingly uneven, and diversified by many an unseen hill and 

 dale. The deep-sea zone appears to have nearly always 

 a soft bottom, consisting of a much finer sediment than 

 that which covers the bed of the coralline zone. The 

 only vegetable organisms which are found in it are tiny 

 and almost microscopic Diatoms. It is inhabited by 

 various kinds of Mollusca, all of which are, probably 

 from necessity, animal-eaters. They appear to exceed in 

 number, as well as in variety, the inhabitants of any of 

 the other zones, judging from the scanty opportunities 

 which occur for investigating the contents of this exten- 

 sive sea-bed. The point of zero in the scale of sub- 

 marine life has not yet been, and perhaps never will be, 

 found. 



