268 F. A. BATHER [ocTOBER 
1. Shore-region, reaches to a depth of 2 or 5 metres, with sandy 
bottom and a vegetation of Ulvaceae, Fucus, Chorda, a number of fine, 
thread-like green algae, Potainogeton pectinatus, and some zostera. For 
the list of characteristic species, reference must be made to the original 
paper. Most of them pass far up into the Baltic. 
2. Zostera-region, usually with sandy bottom and zostera, from 3 
to 15 metres deep. 
3. Alga-region, broadly speaking from 15 to 20 metres deep, 
divided into (a) Laminaria sub-region, usually with a soft bottom of 
mud, often mixed with stones and shells; (4) Furcellaria sub-region, 
forming thick carpets with admixture of various red algae; (c) Coral- 
line sub-region, with calcareous and red algae on a stone bottom. This 
last is more distinct in the nature of the bottom, and has a fauna more 
peculiar to itself, including many chitons. 
4, Deeper water, without vegetation, usually outside the 20 metre 
line, subdivided thus: (a) dead zostera bottom, clayey or muddy with 
many dead leaves of zostera, which give it firmness and serve as food 
for several animals. This usually comes next to the Alga-region, and 
may reach a depth of 33 metres. When the zostera leaves are fewer 
and the clay mixed with sand, it passes over into (2) mixed bottom, 
which often contains many shells in a floor of sand and clay mingled 
in varying proportions ; thus it passes into the two following: (¢) sand 
bottom, often with shells, shell gravel, or shell sand; this is found in 
places where the current is strong enough to sweep away the finer mud, 
which goes to form the chief part of (¢) clay or clay-mud; this, which 
is found in the greatest depths, is loose or oozy, but has no evil 
odour of decomposing organic substances. 
In the Sound these various kinds of bottom do not, as in more 
open seas, succeed one another from shallower to deeper water, but 
depend rather on the currents, so that sand or mixed bottom may be 
found at greater depths than clay or mud. 
From the facts given at length in the original papers it appears 
that almost every species of animal shows a preference for one par- 
ticular kind of bottom. In many cases this is because they are suited 
to a certain mode of life, so that if, after the breeding period, the larvae 
sink on to a spot with unsuitable bottom or where other conditions of 
life obtain, the animals die off at once or in a short time. For instance, 
if mud from the depths be passed through a fine sieve, dead shells of 
young Astarte are often found, sometimes in great numbers. This 
shows that Astarte cannot exist on the soft mud. Its shell is too 
heavy: it sinks and perishes. On the other hand, its thick shell with 
stout epidermis is fitted to withstand rubbing and knocking against 
sand and pebbles, and a bottom of such nature is firm enough to pre- 
vent the shell from sinking into it. Leda, on the contrary, with its 
shell swollen up in front and beak-shaped behind, with its strong foot 
spread out like a sheet, is well equipped for living and boring in the 
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