SYSTEMATIC ACCOUNT 323 



total phytoplankton, where it forms a natural association with certain filamentous 

 species of Fragilariopsis. Much research is required before the true relationship of all 

 the forms is known. The type species, Dactyliosolen antarcticus Castracane, is un- 

 questionably polymorphic, and much structural variation is found upon the connective 

 zones of the several cells of one chain. It seems likely that two distinct groups can be 

 recognized, the cold-water group comprising Dactyliosolen antarcticus and its various 

 forms, and a warm-water group, Dactyliosolen mediterraneus. Chromatophores : 

 numerous rounded bodies; nucleus usually central. 



Dactyliosolen antarcticus Castracane. (PI. VI, fig. i.) 



Castracane, 1886, p. 75, pi. 9, fig. 7. 

 Gran, 1905, p. 25, fig. 26. 

 Karsten, 1905, p. 93, pi. 9, fig. 10. 

 Lebour, 1930, p. 76, fig. 50. 

 Hustedt, 1929, p. 556, fig. 316. 



Cells sometimes solitary, but usually united to form short chains of two to eight cells. 

 Cells cylindrical, straight, valves flat or nearly so. Connective zone composed of 

 numerous intercalary bands so arranged that the ends or seams of the bands form a 

 spiral line about the pervalvar axis on the zone. Each intercalary band bears a line of 

 oval or somewhat elongated puncta. These puncta have one end rounded and the other 

 more or less square, and they decrease in size as the band approaches its oblique ter- 

 mination. Chromatophores: several rounded bodies, nucleus central. Diameter of cell 

 20-64/x ; pervalvar axis up to 130/j.. 



Dactyliosolen antarcticus is a polyphasic species-system having a wide distribution in 

 polar and sub-polar waters. The "antarcticus" phase occurs in both hemispheres and 

 is common in the North Sea, and the North Atlantic. In the southern hemisphere it is 

 seldom recorded north of latitude 40 S. It was observed very frequently, sometimes in 

 great numbers around the South Sandwich Group, in the Drake Strait and particularly 

 in the Southern Ocean a little below latitude 6o° S, to the north of Enderby 

 Land. 



Observed at Sts. 365, 368, 369, 378-388, 453, 659, 661, 664, 666, 670, 1356, 1358, 

 J 359> x 3 6 2. 



"borealis" phase. 



Dactyliosolen borealis Karsten. 

 Karsten, 1906, p. 160, pi. 29, fig. 1. 



The " borealis" phase differs from the " antarcticus " phase only in the markings upon 

 the intercalary bands. The bands are arranged in a similar fashion, so that the ends 

 produce a spiral line upon the zone as in the type phase, but the markings take the form 

 of lines or bars which completely cross the bands, dividing them into square compart- 

 ments. The markings were clear and the zone was usually strongly siliceous. Diameter 

 of cell 36-50/1. 



This phase was observed but seldom and specimens often showed a diphasic tendency, 



