INVERTEBRATE BOTTOM FAUNA 491 



spatangid Schisasier fragi/is, y^h'ich is plentiful in the North 

 Sea, but not found in the Skagerrack/ 



We propose now to discuss the fauna of the continental 

 plateaus within the boreal region, dealing firstly with depths less 

 than 100 metres, "-^ and secondly with depths greater than 100 

 metres. 



I. Continental Plateaus covered by less than 100 Metres (?/ The southern 

 Water. — In the portion of the North Sea to the south of ^"^'^J^J'/JJg 

 the Dogger Bank, where the waters are shallow and the North Sea. 

 summer temperature is high, there are southern forms unknown 

 farther north, though this exclusively southern element in the 

 fauna is very inconsiderable compared with the remaining 

 boreal forms, some of which are more abundantly developed 

 than in more northerly latitudes. During the cruise of the 

 " Michael Sars " in 1904, I was able to carry out investigations 

 with the dredge at a series of stations from the Danish coast 

 to Scotland, in lat. 56° to 58° N. in depths between 14 and 100 

 metres, an area not previously systematically examined. 



The floor of the North Sea is for the most part covered with 

 soft materials (sand, sandy mud, and clay), with areas of stony 

 bottom in places, though even here the rocks and stones are 

 nearly always mixed with softer materials. In some localities 

 the soft materials contain masses of empty shells, which are 

 invaluable to the animal life, acting as a foundation for the 

 hydroids, bryozoans, and other attached forms. This mixed 

 bottom supports a greater variety of forms than the soft bottom, 

 offering suitable conditions to unattached forms, whether they 

 burrow or not, as well as to attached forms. 



The abundance of echinoderms characterises to a great extent 

 the fauna of the North Sea. Among the star-fishes Asterias 

 rubens occurs at all depths and upon every kind of bottom, 

 though it seems less partial to soft clay bottom at considerable 

 depths. Astropecten irregularis is met with everywhere, and the 

 sea-mice Echinocardium and Spatangus purpureus ^ are equally 

 common. Ophiura ciliaris (see Fig. 347) may be described as 

 the brittle-star of the North Sea, for we found well-developed 

 specimens everywhere on mixed bottom down to a depth of about 

 100 metres, and at temperatures varying from 7° to 12° C, but 



1 The continental deep-sea zone not being represented, or only in very limited tracts, in the 

 coastal areas of the Skagerrack, Kattegat, western and southern North Sea, a good many forms 

 characteristic of that zone are absent here. 



- As the type for this area we take the southern and central parts of the North Sea, those 

 parts being the best explored. 



•' In a trawling at 96 metres we found 500 specimens of the last named. 



