1899] THE FAUNA OF THE SOUND 269 
clay: so too is Abra with its thin light shell and long siphon. Cyprina 
also is prevented by its almost ball-shaped shell from sinking in the 
clay; at the same time it prefers a bottom mixed with sand. The 
long arms of the sandstar, Amphiura, and the felted spines on the 
under side of the heart-urchin, Echinocardium, must also bear up the 
animal’s body on a loose bottom. NVatica, which burrows with its out- 
spread foot, has not much to fear from clay, though it usually prefers 
some other kind of floor. Buccinum thrives in clay: it is strong 
enough to work itself along there. Cardiwm fasciatum is found on all 
sorts of bottom. But the animals that do best in the clay mud are a 
number of Chaetopods. When, however, the clay is made firmer by 
admixture of sand, or by a carpet of dead zostera leaves, a far richer 
fauna is able to develop. 
Difference of depth has here scarcely any effect on the distribution 
of species, since the whole Sound is so shallow that it would come 
within the littoral zone as usually understood. Such difference as 
there is has an indirect influence through its effect on the water. The 
southern sill and the narrowing between Saltholm and Scania cause 
the brackish currents from the Baltic to reach right to the bottom ; 
but as the Sound widens again these currents broaden and thin out, 
so that their effects do not stretch so deep. Thus the bank between 
Malmé6 and Saltholm forms a complete barrier against the marine forms; 
the southern end of the Sound is occupied by a brackish water fauna, 
and the limit between this and the deeper salt water fauna gradually 
rises nearer the surface as it approaches the northern end of the Sound. 
The: southerly increase of conditions unfavourable to a purely marine 
fauna differentiates the whole fauna into four classes according to the 
distance to which each penetrates the Sound. 
We are now in a position to discuss the origin of the fauna of the 
Sound. We have seen how, in class after class, the species of purely 
Arctic or partly Arctic distribution outnumber those with a west 
European or more southern range. We have noted also that the pro- 
portion of northern forms is greater in the Sound than in neighbouring 
seas. Further than this, there are in the Sound a number of northern 
species which are not found in the Kattegat at all, or only in its most 
southerly portions, or which, if they do oceur over the whole Kattegat, 
are not found in any quantity till one comes south. The Echinoderms 
furnish specially good examples. The holothurian, Phyllophorus 
pellucidus, is fairly common in the Sound, but only one specimen has 
ever been taken in the Kattegat, and that was in its southerly ex- 
tension. It is not known off the more northerly Bohustiin. But this 
Species is typically Arctic ; it occurs in the Norwegian Finmark and at 
Spitzbergen, and specimens found there cannot be distinguished from 
those dredged in the Sound. On the other hand, the Phyllophorus that 
occurs off western Norway, as well as the allied English form, both 
differ from that of the Sound. Phyllophorus drummondi, also taken in 
18—war. sc.—vou. xv. No. 92. 
