ddl 
ON SHELLS FLOATING ON THE SURFACE OF THE SEA. 
By Aveusr Kroeu, M.Sc. (Copenhagen). 
(Communicated by A. C, Johansen, M.Sc.) 
Read 3rd April, 1908. 
In a paper ‘‘On the hypotheses on the sinking of sea-beds, based 
on the occurrence of dead shallow-water shells at great depths in the 
sea,’”’’ Mr. A. C. Johansen discussed the agencies by which the shells 
of Mollusca may be transported from the places where the animals 
lived, and spread over the bottom of the ocean (p. 427). Among 
these are mentioned :—‘‘ Marine surface currents. These transport 
(a) floating ice, (6) seaweeds to which molluscs are fixed, (c) the 
molluscs themselves.” 
The evidences for this last method of transportation (c) were the 
following (pp. 429-30) :— 
I. On the occasion of a determination of the specific gravity of 
some shells Mr. Johansen and I noticed that some specimens of 
Mytilus edulis, taken alive, but kept dry for a couple of days, were 
able to float. One of them floated for more than 24 hours. 
II. In an experiment with another species of Mytilus Mr. Johansen 
found that some small dead specimens floated for several days. 
Ill. The Danish East Greenland Expedition of 1892 actually 
obtained one specimen of Mytilus edulis, 11 mm. in length and probably 
dead, floating in the open ocean (lat. 75° 37’ N., long. 6° 40’ W.}. 
In another paper* Mr. Johansen recorded (pp. 15-16) similar 
occurrences in fresh waters. He found shells, filled with air, of 
Bithynia tentaculata, Valvata- cristata, and several species of Limnea 
and Planorbis floating about in great numbers in lakes, and he mentions 
that Dr. W. Sorensen has several times observed shells of Planorbis 
corneus, occupied by Argyroneta, floating about in ponds in springtime. 
Possibly the instances above set forth—and they are all that 
Mr. Johansen has been able to find—will be thought to be somewhat 
meagre, though it should be borne in mind that the chance of obtaining 
specimens by tow-netting must be extremely small, even if their total 
number a year is rather considerable ; the more so, as the shells will 
float on the very surface of the water, and most tow-netting is carried 
on at a depth of at least some inches. 
1 Vid. Medd. Naturh. Foren. Kjébenhavn, 1902, pp. 393-435. 
2 «(Qm Aflejringen af Molluskernes Skaller i Indséer og i Havet’’ (‘On the 
Deposition of Shells in Lakes and in the Sea’’): Vid. Medd. Naturh. Foren. 
Kjobenhayn, 1901, pp. 5-46. 
