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, Arn/phiura, and the felted spines on the 

 under side of the heart-urchin, FJchinocardium, must also bear up the 

 animal's body on a loose bottom. Natica, 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. Cardium 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 

 Malmo 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 occur over the whole Kattegat, 

 are not found in any quantity till one comes south. The Echinoderms 

 furnish specially good examples. The holothurian, Phyllophorus 

 pcllucidus, 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 Bohustan. But this 

 species is typically Arctic ; it occurs in the Norwegian Finmark and at 

 Spitsbergen, 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 NAT. SC. VOL. XV. NO. 92. 



