322 DISCOVERY REPORTS 



Genus Leptocylindrus Cleve 

 Cleve, 1889 



A small genus with a true plankton habit. It is spread widely throughout the northern 

 hemisphere and is represented in freshwater lakes as well as in the sea. It belongs to 

 the "solenoid" group of diatoms, but unlike Rhizosolenia, the valves do not possess a 

 mucro or any form of process. The development of the cell in the pervalvar axis is by 

 means of numerous intercalary bands, but owing to the weakly siliceous nature of the 

 cell wall, they are seen with difficulty. The cells are united by the valves to form chains 

 or filaments, which are usually straight. Chromatophores : usually two, but may be 

 numerous; nucleus central. 



Leptocylindrus danicus Cleve. (PI. XI, fig. 6.) 



Cleve, 1889, p. 54. 



Meunier, 1910, p. 258, pi. 28, figs. 31, 32. 



Gran, 1905, p. 24, fig. 24. 



Hustedt, 1929, p. 558, figs. 318, 319. 



Lebour, 1930, p. 77, fig. 52. 



Cells tubular, cylindrical, narrow, straight, united to form straight chains. Valves 

 circular, flat or slightly convex, without mucro or any visible structure. Connective 

 zone elongated, and bearing numerous pointed segments which fit together by their 

 apices giving the connective zone an appearance of being composed of intercalary scale- 

 like markings similar to many species of Rhizosolenia. Chromatophores: numerous 

 cocciform bodies. Diameter of cell 5—18^ ; pervalvar axis 30-65^. 



Probably a neritic species, but often found under oceanic conditions. It is spread 

 widely throughout northern European waters and is encountered sometimes in enor- 

 mous quantities. It was observed, but in small numbers only, in the Drake Straits and 

 off the Cape of Good Hope. It occurred also in the Peru Current material, where the 

 specimens were very much larger than those observed at the other two stations. Those 

 from the Drake Straits were particularly small. 



Observed at Sts. 381, 428; WS 710. 



Genus Dactyliosolen Castracane 

 Castracane, 1886 



The genus Dactyliosolen is closely allied to Leptocylindrus and with it forms a sharply 

 defined family. The "solenoid" formation of the cells reaches its highest expression in 

 this family, and as the cells are perfectly symmetrical about the pervalvar axis, it is the 

 furthest removed from the gonioid influence of the Biddulphiaceae. The valves are circular 

 or discoid, and bear neither spine nor process. The zone of Dactyliosolen is composed 

 of numerous and probably discontinuous annular imbricate segments which appear 

 often to invest the cell with a certain spiral torsion. 



It is probable that all the forms are truly oceanic. The genus is represented in both 

 hemispheres, and in the sub-Antarctic Zone constitutes a considerable proportion of the 



