176 THE OCEAN 



small particles of organic matter there settling 

 on the bottom. This is called the mud-line, 

 and on shores facing the great oceans its 

 average depth is about 100 fathoms. This 

 links on to a sort of artificial bottom in the 

 open ocean, where a marked change in viscosity 

 occurs, and a consequent change in the rate 

 of fall of particles of organic detritus (see 

 Chapter VII., p. 138). 



Benthos of the Continental Slope. — The fauna 

 living on the continental slopes beyond 100 

 fathoms is sometimes called the archibenthal 

 fauna, but there is no clearly defined boundary 

 between the archibenthal and abyssal areas. 

 This archibenthal fauna in the North Atlantic 

 is characterised by sea-urchins of the family 

 Echinothuridae, with flexible leather-like shells, 

 and other echinids, crinoids, starfishes, brittle- 

 stars, holothurians, actinians, pennatulids, 

 free corals, alcyonarians, crustaceans, worms, 

 molluscs, brachiopods and sponges. The fish- 

 fauna living on the continental slope of the 

 Eastern Atlantic was found during the 

 " Michael Sars " Expedition to be very uniform 

 all the way from the Faroe Islands to south of 

 the Canaries, six species being common to these 

 northern and southern localities (Mora mora, 

 for example), thus differing from the fish-fauna 

 of the continental shelf, the species of which 

 are much more limited in their distribution. 



