514 DEPTHS OF THE OCEAN chap. 



That there must have been considerable alterations in the 

 physical conditions of the sea on these banks appears evident 

 from the large decayed shells of an arctic form, Pecten islandicus, 

 and the remains of other arctic molluscs. The enormous 

 quantities of empty shells of more southern forms may indicate 

 that special forces have been at work, resulting in the destruc- 

 tion of these animals in vast numbers. But, on the other hand, 

 I consider it too hasty an assumption from a biological point of 

 view to maintain that, because these forms are in other localities 

 solely or mainly littoral forms, their extinction must have been 

 due to subsidence of the ocean-floor. As already mentioned, 

 the " Michael Sars " dredged from the bank large living 

 specimens of several of the species represented by empty shells 

 in such abundance, showing that there is still a possibility of 

 finding the necessary conditions of existence there. And there 

 were also some characteristic littoral forms, like Echinus 

 esculentus, Opkiiira albida and Alcyonhmi digitahtm, of which 

 the first named was in too great abundance to have been 

 merely the result of chance. 



The occurrence of these forms may perhaps be explained 

 by the high temperature (9.33°C.) at these depths in the middle 

 of August 1902 — a temperature differing very slightly from 

 that prevailing at the same season along the Norwegian coast 

 in the shallower depths principally inhabited by these forms — for 

 temperature and salinity more than depth regulate distribution. 

 An extinct fauna of forms like these at a spot somewhere 

 out on the plateaus does not necessarily imply subsidence of 

 the bottom, but more likely physical changes in the sea-water. 

 Oysters and many other forms are examples of this. A further 

 instance may be cited from the North Sea cruise of the 

 " Michael Sars" in 1904. At Jammer Bay, on the north-west 

 coast of Jutland, at a depth of 14 metres, the dredge brought up 

 great quantities of Mactra elliptica, Lunatia intermedia, 

 Ophtura ctliaris, Echinocardiiim, etc., along with a very large 

 number of empty shells belonging to the mussel Venus gallina, 

 of which only two living specimens were found. It would be 

 absurd to assert in this case that mortality was due to changes 

 of level, as this form is found elsewhere in quantities at such 

 depths, but the numbers of empty shells point to an encroach- 

 ment of unfavourable conditions. Another factor must be 

 kept in view, namely bottom-currents, that may possibly, under 

 certain circumstances, accumulate bottom-material such as piles 

 of empty shells at particular localities, which would not 



