STYLASTKRIDAE 25 



Stylaster roseus forms a parallel to Pliobothrus symmetrzcus. It is fairly common between 230 

 and 620 metres at Florida and on the whole has probably a greater bathymetric distribution than 

 Pliobothrus symuietricus. Stylaster roseus is much more frequent in its occurrence than the latter form, 

 bnt is restricted in the Atlantic to the south of the submarine ridges and has not yet been found in 

 the Norwegian Sea. The Ingolf« has taken the species at a single place in the Denmark Strait in 

 water of negative temperature; this occurrence is due probably to a submarine wave making the 

 conditions inhabitable for the species when it became attached as larva or that the station at the time of 

 observation was covered by a wave of the cold polar water -- In the Norwegian Sea the species is 

 replaced by the nearly allied Stylaster gemmascens^ which in reality must be ranged with the extremely 

 few animals, which are entirely bound to the warmer layers of the Norwegian Sea. We may feel tempted 

 to consider it a biologically defined, local species, which has divided off from Stylaster roseus. The 

 two species have only been found side by side with certainty at the above-mentioned boundary station 

 in the Denmark Strait, where the line of separation must be drawn between the Atlantic deep-water 

 region and the boreal water-layers. This is the only time that Stylaster gemmascens has also been 

 found in water of negative temperature. Once the species has been identified with certainty south of 

 the Wy ville-Thomson ridge, a couples of colonies being found at Rockall ; the fauna at Rockall however 

 has a strong mixture of species, whose chief occurrence is bound to the Norwegian Sea. 



Finally, the last species Stylaster norvegicus is an Atlantic species which belongs to the North 

 Atlantic and has been able to penetrate into the Norwegian Sea, where it has found a new home in 

 the warmer water-layers there. Its occurrence shows a secondary centre in the Troudhjem Fjord, 

 where along with Stylaster gemmascens it is more abundant than anywhere else in the northern parts 

 of the Atlantic. The occurrence of the species in more southern waters cannot be accepted as certain, 

 for its systematic characters have hitherto been too little unravelled; but it can hardly be very 

 common there. 



Trondhjem, November 1913. 



The Ingolf-Expedition. V. 5. 



