348 DISCOVERY REPORTS 



Kiitzing himself used Sigmatella as a subgenus of Friistulia, in his Synopsis Diatomearum 

 (18336) and described a number of organisms under that name which undoubtedly were 

 not congeneric ; for instance, Friistulia Nitzschii, illustrated in pi. 14, fig. 33a, b, is what 

 is known to-day as a Nitzschia, and Friistulia attenuata in pi. 14, fig. 35 a, b, is a 

 Pleurosigma or Gyrosigma. It is most likely that it was to this publication (Kiitzing, 

 18336) of Sigmatella that Hassall referred when he said that Gyrosigma and Sigmatella 

 were synonymous, meaning that the organism illustrated on pi. 14, fig. 35 a, b, was 

 synonymous with Gyrosigma, and not the whole of the genus Sigmatella as described 

 in 1833a. However, Hassall gives no direct indication to which mention of Sigmatella 

 Kiitzing he referred, and, strictly speaking, it is very doubtful whether such taxonomic 

 considerations can be allowed to affect the position of the names. 



In such an atmosphere of doubt I have decided to use the generic name Pleurosigma 

 in the sense that Wm Smith used it in 1853. The name Gyrosigma has been used by 

 Cleve (1894-5), Gran (1905), Lebour (1930) and Hustedt (1930). Hustedt followed 

 Cleve and used the name in a special sense : Gyrosigma was used to designate those forms 

 in which the longitudinal striations were in the apical axis of the cell and crossed the 

 transverse striations at right angles, while Pleurosigma was used to designate those forms 

 in which the striations in the apical axis were crossed by oblique striations. Personally 

 I do not think that the differences involved warrant generic distinction. 



Pleurosigma directum Grunow. 



Cleve and Grunow (1880), p. 53. 

 Karsten, 1905, p. 127, pi. 18, fig. 5. 



Cells solitary. Valves flat or nearly so, rhombic-lanceolate to elliptic-lanceolate in 

 outline. Raphe distinct, only very slightly sigmoid, central knot very small. Striation 

 consisting of two systems of lines of faint puncta which cross at an angle of about 6o°. 

 Chromatophores : two anastomosing bands; nucleus large, central. Apical axis of cell 

 1 80-270 ju, ; transapical axis 44/^.. 



This characteristic species is seldom found in large numbers, but has a wide distri- 

 bution in the waters of both the north and south polar seas. 



Observed at St. 428, 437, 478. 



Pleurosigma directum-secundum Karsten. 

 Karsten, 1906, p. 175, pi. 34, fig. 6. 



This species is accepted only with great diffidence. It differs in few respects from 

 Pleurosigma directum Grunow and was found at one station only. The most important 

 difference between it and Grunow's species is in the form of the chromatophores. In 

 Karsten 's species they take the form of numerous short vermiform bodies, while in the 

 other they consist of two irregular bands. Only a few isolated specimens were observed, 

 and I strongly suspect that the polychromatophoric specimens were a degenerate state 

 of Pleurosigma directum. 



Observed at St. 438. 



