400 
ments on the occurrence of specimens at unusual great depths can 
oiily refer to dead sliells. 
The tliird of the explanations which from a theoretical point 
of view can he offered to throw light upon the phenomenon that 
dead shells are deposited far oiitside the vertical range of the species 
to which they helong is, as hefore 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 faet 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 espéces de Mollusques aretiques trouvées dans 
les grandes profondeurs de l’ocean Atlantique intertropicaP. Comptes 
Eendus Séanc. Acad. Sciences, T. 97. Paris 1883. p. 1497—99. 
2) W. H. Dall: „A. Preliminary Catalogue of the Shell-bearing Marine 
Molluscs and Brachiopods of the South-Eastern Coast of the United 
States“. Bulletin U. S. Nat. Museum. Washington 1889. p, 11. 
A. Lo c ard: „Sur l’aire de dispersion de la faune malacologique des 
grands fonds de l’ocean Atlantic boreaP. Comptes Rendus Acad. 
Sciences. T. 126. Paris 1898. p. 441—443. 
C. G. Joh. Petersen: 1. „De skalbærende Molluskers Udbrednings- 
forhold^ etc. Kjøbenhavn 1888. (Dr. Joh. Petersen has noted that 
some shallow-water species reach far greater depths in the actual Baltic 
than in the Kattegat.) II. „General Results„Hauch“s Togter. 
Kjøbenhavn 1893. 
®) A. S. Jensen: „Studier over nordiske Mollusker^. II. 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 mollusc-species 
have not the same vertical range in the Atlantic as in the North 
Polar Basin.) 
