1904 - 5 .] Study of the Lakes of Scotland and Denmark. 439 
by the operations of the abundant littoral fauna, which feeds alike 
on the living vegetation and on the decayed matter ; a large part 
of these passes through the alimentary canals of animals, and is 
transformed into excrementa. The animals which cause this 
transformation are not the same as those found in deeper water, 
but consist mostly of insect larvae and molluscs ; very often we 
find the bottom covered with long greyisli-white excrements of 
snails, especially Limncea auricularia, ampla , and ovata. 
In our lakes the space between the 16-feet and 30-feet contours 
is marked by a remarkable and often very conspicuous elevation 
of the bottom. Explorations show that in two of the lakes at 
some distance from shore a series of banks occur, consisting chiefly 
of mollusc shells embedded in a bluish-grey lake-marl. There is 
no doubt that the molluscs here act as reef-forming factors, and 
it will be understood that in our lakes the molluscs must act as 
such. In the Danish lakes molluscan life (except Pisidnum) does 
not extend beyond the 30-feet contour. The shells in the vegeta- 
tion zone are in great measure dissolved or pulverised by the 
powerful action of the various erosive agencies of this zone. In 
the zone occupying the space between the vegetation zone and the 
outer limit of molluscan life on the lake-floor the erosive power 
of these agencies is much diminished, and in the deeper part of 
the zone almost nil. In the tranquil water here the accumulation 
of shells may go on undisturbed by the grinding and dissolving 
forces, and thus banks of mollusc shells are formed. These banks 
consist of the shells of those mollusca which can live outside the 
vegetation zone, especially Vatvata piscinalis, Bithynia , Anodonta , 
and Vnio, but only to a slight extent of the shells of Limnaa 
and Planorhis , which live mostly in the vegetation zone. The 
accumulation of shells in the “shell-zone” is often enormous, and 
apparently there is a striking disproportion between the large 
amount of empty shells and the relatively few specimens of 
living molluscs ; yet it must be remembered that vast accumula- 
tions of shells may result from a slow process of deposition during 
long periods of time, as from a more rapid deposition during a 
shorter period. 
Inside the shell-zone and closer to the shore we often find more 
local and very peculiar formations, among which may be mentioned 
