INVERTEBRATE BOTTOM FAUNA 533 



and deeper-living ^ boreal forms that are never met with at 

 Spitsbergen. 



Another boreo-arctic area lies in the south-western portion 

 of the Norwegian Sea on the ridge connecting Iceland and the 

 Faroes. The crest of the Wyville Thomson Ridge between the 

 Faroes and Shetland has not been examined by the "Michael 

 Sars," but undoubtedly it may be included. On the broad ridge 

 between the Faroes and Iceland we took up several stations 

 in 1902, at a depth of 450 to 480 metres, the temperature varying 

 between 3.12° C. and 3.98° C. ; the greatest depth on the top of 

 this ridge is about 500 metres. Here we came across the same 

 mixed fauna already described as characteristic of the north- 

 eastern boreo-arctic area, the "Michael Sars" securing 

 distinct arctic forms,- together with boreal forms ^ which 

 penetrate into the boreo-arctic portion of the Barents Sea. If 

 we remember that the polar and Atlantic currents meet about 

 the middle of the Iceland-Faroe ridge, it will be easy to 

 understand the boreo-arctic character of the bottom fauna. It 

 is remarkable that such distinctly cold-water forms as Hyme7iaster 

 and Nymphon robtistum were found in water with a temperature 

 of 3° or 4° C. ; no doubt the individuals were few (only one 

 specimen of Nymphon robushim, for instance, being taken), still 

 their occurrence seems to show that the bottom-water on the 

 ridge has not always the high temperatures we recorded — the 

 temperatures must often be considerably lower, perhaps even 

 below 0° C. at times.* Boreal deep-water forms are furthered in 

 their advance occasionally by warm currents, and yet they can 

 endure low and varying temperatures ; the converse probably 

 holds good with various purely arctic forms, which owe their 

 distribution to the cold arctic water, but can endure the higher 

 temperatures when that is displaced by Gulf Stream water. 

 In spite of this Hymenaster and Nyuphon robusttwi are just as 

 much arctic forms 2isHippasterias, Pentagonaster, diwd Pontophilus 

 are boreal forms. 



^ Antalis entalis, Schizaster fragilis, Hippasterias plana, Pentagonaster granularis. 

 Verruca strovii, Hippolyte securifrons, Crangon alhnanni, Nephrops norvegicus (?), Fottiophilus 

 norvegicus, Munida negosa, and several others. The hydroids, on the other hand, are very widely 

 distributed, as most of the species met with in these tracts are commonly distributed throughout 

 the boreal region ; some species of hydroids seem able to adapt themselves to all temperatures 

 (eurythermal forms). 



- Hymenaster pellucidus, Solaster squamatus, Antedon esckrichti, Rhachotropis aculeata, 

 Epimeria loricata, Nymphon robustum, Lampra purpurea. 



^ Hippasterias plana, Pentagonaster granularis, Schizaster fragilis, Antedon tenella, 

 Gorgonocephalus lincki and G. lamarcki, Pontophilus 7wrvegicus, Sabinca sarsi, and amongst 

 hydroids Thujaria thuja and Hydrallmanuia falcata, although not in any great quantities. 



•* The Danish " Ingolf " Expedition recorded a temperature of +0.5° C. at about 510 

 metres. 



