1508] Decapod Crustaceans. DD 
the fresher surface-water enters as an undereurrent, to be retained 
in the fiord; where it aceumulates in the deeper portions inside the 
barriers, just exactly where the large masses of Pandalus borealis 
are to be found. This salter mass of water, whose salinity is 34 
—35 %0, IS the remains of the winter inflow and accordingly in 
summer it is considerably colder (only about + 5 to 69 C.) than 
the fresher upper layer (about + 109 to 1509 C.). 
The water which flows in over the barriers from the Skagerack 
into the deeper portions of the fiord enters in larger quantity during 
the winter than during the summer months, and it is mainly then 
that the bottom water in the deep basins is renewed. In the winter 
season these masses of water rise higher up towards the surface 
partly because of the force of the inflowing current and partly be- 
cause the new water is a little heavier than the deep-lying masses 
already there, which are now accordingly displaced and lifted towards 
the surface. 
We thus get in a fiord of this kind å regular seasonal variation. 
The deep-lying masses of water with å high salinity (84—3835 90) 
and å temperature about 6% (sometimes as low as about 49 C. and 
sometimes about 79 C.) are nearer the surface in the winter and 
aecordingly oceupy shallower parts of the fiord than they do in the 
summer; whereas conversely in the summer months the same layer 
will not be met with till we come down to the deeper portions of 
the fiord. 
We see then that these seasonal changes in the vertical distri- 
bution of the deep layer correspond with the seasonal variations in 
the oceurrence of Pandalus borealis at different depths. 
Seeing that previous to the institution of prawn-trawling the 
Drammen Fiord was the only locality in which we were aware that 
Pandalus borealis was found in abundance, and that the species was 
otherwise best known from the sub-arctic regions; it was generally 
surmised that like many other animal-forms, such as those found in 
the deeper portions of the Baltie, Pandalus borealis existed in the 
Drammen Fiord merely as å survival from the Glacial Period which 
had been cut off from its natural haunts. In view however of recent 
hydrographical biological investigations, which have largely enriched 
our knowledge of the cireulation of the water-layers and their in- 
fluenee upon the distribution of organisms, it will hardly be possible 
any longer to defend this assumption, at least so far as Pandalus 
borealis is concerned. 
