490 DEPTHS OF THE OCEAN chap. 



comprises quite a number of shallow-water forms that are 

 otherwise foreign to northern regions — Mediterranean immi- 

 grants which make occasional visits or have effected a 

 permanent lodgment in comparatively limited tracts. Some of 

 them I shall refer to later on, when dealing with the shallower 

 portions of the North Sea. Their presence may be ascribed to 

 hydrographical conditions, and in no way depends upon the 

 topography of the bottom. To some extent the English 

 Channel acts as a boundary between two littoral faunal areas, a 

 fairly large number of Mediterranean forms living in the 

 Channel but not venturing into the North Sea ; while on the 

 other hand several northern forms do not enter the Channel, 

 these last being especially forms of Arctic origin. Many or 

 probably most of the species are common to both areas, since the 

 majority of the boreal species of the North Sea were originally 

 immigrants from southern waters. 



So far as the coasts of the boreal region are concerned the 

 sublittoral zone does not vary much, though certain species from 

 the continental deep-sea zone, which ascend to the sublittoral 

 zone along the North Sea and Atlantic coasts of Scandinavia, are 

 absent from large portions of the Skagerrack and Kattegat as well 

 as from other coasts of the North Sea. They would seem to be 

 forms whose distribution follows the Gulf Stream, and are there- 

 fore found mainly along the eastern coasts of the North Sea 

 and Atlantic. They include the holothurian Psohis sq2ianiatus, 

 the asterid Pentagonaster granularis, the gephyrean Bonelha 

 viridis, the brachiopod Waldheimia cranium, and some mussels. 

 Munida rugosa, which is one of the most characteristic decapods 

 belonging to the sublittoral and deep-sea zones is, according to 

 Theel, seldom met with on the Bohuslan coast of Sweden ; the 

 true corals and gorgonids of the deep-sea fauna, which else- 

 where patronise the sublittoral zone, are much restricted in 

 their distribution throughout the Skagerrack and wide tracts of 

 the North Sea, and seem to be absent from the fjords of the 

 Bohuslan coast. Certain forms, which along the coasts are 

 chiefly sublittoral in their distribution, occur sometimes quite 

 commonly in one area, whereas in another area they may be 

 scarce or even entirely absent. For instance, on the Swedish 

 and Norwegian coasts of the Skagerrack the spatangid 

 Brissopsis lyrifera is generally met with in the sublittoral 

 zone, but on the west or North Sea coast of Norway it is 

 comparatively rare. The converse is the case with the 



