knowledge of the conditions of life of the marine
mollusca and of their distribution is extremely scanty. We are apt to
imagine that the bottom of the sea is covered by a more or less uniform
thick layer of shells; but whenever a careful survey of the nature of
the deposits now forming there has been made, such is by no means found
to be the case. Some of the best results obtained by that useful body,
the Liverpool Marine Biological Committee, have been precisely in this
direction. A most interesting account has been published by Professor
Herdman and Mr. Lomas on the floor deposits of the Irish Sea, in which
the authors state (p. 217), that "a place may be swarming with life and
yet leave no trace of anything capable of being preserved in the fossil
state, whereas in other places, barren of living things, banks of
drifted and dead shells may be found, and remain as a permanent deposit
on the ocean floor."

Owing to the fact of the peculiar geographical position of Scandinavia
at this time--an isthmus of land with a high mountain range lying
between the warm Atlantic and the cold Arctic Sea--the snowfall must
have been excessive, and large glaciers were evidently forming. These
produced icebergs as soon as the lower parts had advanced to the Baltic
coast-land and deposited their detritus in the sea. Immense masses of
mud and stones were thus cast to the bottom of the sea, and under these
circumstances no delicate mollusca or other marine life probably could
have developed within a considerable distance from the shore. To judge
from the direction pursued by the majority of the boulders from their
source of origin, the prevailing current during the deposition of the
lower boulder-clay was from north-west to south-east. It is possible
that little marine life, except free-swimming forms, would have been
able to live within the Russian area of this sea. But the free-swimming
larvae of molluscs and other surface species were not prevented from
passing from the White Sea south-westward, and in sheltered localities
where little or no mud deposition was going on, these no doubt might
have developed into adults on the sea-floor. It is quite conceivable,
therefore, that in one portion of the North European Sea, which was
fully exposed to the destructive influences of the iceberg action, the
fauna was scanty or totally absent, while in another part there lived a
fairly abundant one. The unfossiliferous state of the lower continental