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face, it is not easy to imagine in what way the marine molusca 

 can be affected by pressure. It is evident that they do not 

 require the protection of stronger shells, as these are generally 

 thinner and more delicate in deep than in shallow water. 

 There are, doubtless, however, other conditions which must 

 render the depths of ocean unfavourable to the development 

 or existence of animal life ; perhaps the most important of 

 these may be the total absence of light ; and the fact, noticed 

 by travellers, of the extraordinary transparency of the Norwe- 

 gian seas, may explain why molusca frequent greater depths 

 there than in other parts, as is said to be the case. 



" The most general characters of molusca obtained from a 

 great depth, compared with individuals of the same species 

 inhabiting shallow water, consist in smallness of size and defi- 

 ciency in colour. There are also the modifications of form 

 already alluded to, which I have brought a few specimens to 

 illustrate — all from the coasts of Zetland — viz., the common 

 whelk (buccinum undatum), from less than 10 fathoms, the 

 same from about 25 fathoms, and again, from 50 or 60 fathoms. 

 It will be observed, that the form becomes more elongated the 

 greater the depth of habitat, which appears to be a law apply- 

 ing only to the particular species. In fusus corneus the very 

 reverse occurs, as it is found invariably shorter in proportion 

 to the greater depth it frequents. The specimens are from 

 7, from 30, and from 80 fathoms. I have also brought, for 

 comparison, specimens of two common bivalves — venus gallina 

 and venus ovata — obtained in shallow water and at a depth of 

 80 or more fathoms. 



" After further experience, I see no reason to change the 

 opinion I expressed on a former occasion respecting the rela- 

 tion existing between latitude and depth in the sea, not being 

 analogous to that which is found to exist between latitude 

 and elevation on land. 



" It is well known that the vegetation of the arctic regions 

 is repeated at great elevations in temperate and tropical lati- 



