544 DEPTHS OF THE OCEAN 



tribution of the Atlantic deep-sea^ fauna, that some of the 

 forms occur likewise in the deeper parts of the boreal areas of 

 the Norwegian Sea. This, however, refers only to a small 

 proportion, since the majority consist of specifically Atlantic 

 forms which do not cross the boundaries of the Norwegian 

 Sea. As to the distribution of this specifically Atlantic fauna 

 opinions differ. One very prevalent view is that, throughout 

 the North Atlantic at any rate, temperatures, salinities, and 

 other external physical conditions are extremely uniform, and 

 that consequently the various forms have a correspondingly 

 extensive distribution. Certain facts seem to me to contradict 

 this, for instance, in such well-known groups as the echinoderms 

 and decapod crustaceans, of which there are numbers of species. 

 Mortensen's work on the North Atlantic echinids, and Koehler's 

 description of the material collected by the Prince of Monaco, 

 show that the West African coastal seas shelter 28 species of 

 echinids, and that immediately to the south of the ridges 21 

 species of the same group have been trawled by the " Ingolf " 

 and "Michael Sars." In all these two areas yielded 39- 

 species, but not more than 10 of them are common to both. 



We find much the same position of affairs when we compare 

 the deep-sea fauna of the European or African Atlantic 

 side with its counterpart on the West Atlantic (American) 

 side.^ Merely taking the echinids, which may be regarded as 

 specifically belonging to the archibenthal-abyssal fauna on both 

 sides, there are altogether 74 species, but only 24 of them are 

 common to both areas. The other groups of echinoderms 

 have not yet been so carefully studied, but we know enough 

 to show that in their case, too, a similar difference exists between 

 these archibenthal-abyssal areas of the Northern Atlantic. 



If we take decapod crustaceans the result is still the same. 

 The northernmost portion of the European Atlantic area 

 immediately south of .the ridges has been examined by Danish 

 and Norwegian expeditions at many stations, and 15 archi- 

 benthal-abyssal species of Brachyura and Anomura have been 

 discovered at depths of 1000 to 2000 metres, while the 

 researches of the Prince of Monaco, and the " Travailleur '" and 

 " Talisman " Expeditions, have resulted in 40 species being 

 found at the same depths in West African Atlantic waters ; 



1 I wish to make it clear that in what follows no distinction will be made between the archi- 

 benthal and abyssal faunas, unless expressly stated, but would merely remark that the bulk of 

 the species belong to the archibenthal zone. 



- I have omitted one or two species that have a very extensive bathymetrical distribution, 

 inasmuch as they occur also in the littoral and sub-littoral zones of the coastal areas. 



* No account has here been taken of pelagic deep-water forms. 



