268 F. A. BATHER [october 



1. Shore-region, reaches to a depth of 2 or 3 metres, with sandy- 

 bottom and a vegetation of Ulvaceae, Fucus, Chorda, a number of fine, 

 thread-like green algae, Potamogeton 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 ; (&) 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 conies 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 (b) 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 : (c) 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 (d) 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 



