400 
ments on the occurrence of specimens at unusual great depths can 
only refer to dead shells. 
The third of the explanations which from a theoretical point 
of view can be offered to throw light upon the phenomenon that 
dead shells are deposited far outside the vertical range of the species 
to which they belong is, as before said, that the species at an earlier 
period, in some parts of the sea, may have had another vertical 
distribution than they have at present. It is not. without interest 
to discuss this explanation. There is no doubt that the vertical 
distribution of many mollusc-species depends very much on the hydro- 
graphical' conditions. Many authors, e. g. P. Fischer!), W. H. 
Dall?), A. Locard?), C.G. Joh. Petersen”) and A.S. Jensen?) 
have drawn the attention to the fact that mollusc-species have not 
the same vertical range in places where the hydrographical condi- 
tions are different. There is e. g. for many species a very great 
difference in their vertical distribution in the North Polar Basin, 
1) P. Fischer: ,Sur les espåces de Mollusques arctiques trouvées dans 
les grandes profondeurs de V'ocean Atlantique intertropical”. Comptes 
Molluscs and Brachiopods of the South-Eastern Coast of the United 
States". Bulletin U. S, Nat. Museum. Washington 1889. p. 11. 
2) A. Locard:. ,Sur Taire de dispersion de la faune malacologique des 
grands fonds de Tocean Atlantic boreal”. Comptes Rendus Acad. 
Sciences. T. 126. Paris 1898. p. 441—443. 
1") C.G. Joh. Petersen: I. ,De skalbærende Molluskers ger er 
salg ete. Kjøbenhavn 1888. (Dr. Joh. Petersen has noted that 
me shallow-water species reach far greater depths in the SR alle 
ER in the Kattegat.) Il ,General Results". ,Hauch"s Togter. 
Kjøbenhavn 1893. 
5) A. S. Jensen: ,Studier over nordiske Mollusker". IL. Vid. Medd. 
Naturh. Foren. Kjøbenhavn 1901. p. 38. (Mr. Jensen mentions in 
this paper that Cyprina islandica reaches greater depths in the Atlantic 
than in the White Sea; in a letter he has informed me that he also 
in several other cases has made the observation that molluse-species 
have not the same vertical range in the Atlantic as in the North 
Polar Basin 
