236 DISCOVERY REPORTS 



form. It is very similar to H. chromatoaster , but usually a little larger and more robust. 

 The chief differences lie in the form and arrangements of the chromatophores and in the 

 orientation of the moniliform striae. A neritic diatom characteristic of Antarctic Seas. 

 Observed at St. 482. 



Hyalodiscus stelliger Bailey. 



Bailey, 1855, vol. vn, p. 10. 

 Lebour, 1930, p. 30, figs. 10, 11. 



Cells discoid, valves strongly convex, finely striate, striae moniliform. Central area of 

 valve strongly differentiated, peripheral zone striate, striae often parallel or nearly so, 

 fasciculate. In central area, puncta few, irregularly arranged. Girdle composed of 

 narrow intercalary bands. Chromatophores: numerous small plates. Diameter of 

 valve 27-80/n ; pervalvar axis 12-35/M. 



Neritic and meroplanktonic, may be tychopelagic. Not common in the southern 

 hemisphere, a few specimens only being observed off the Brazil coast. Widely distri- 

 buted in northern waters and the Mediterranean Sea. 



Observed at Sts. 719, 721. 



Subfamily SKELETONEMOIDEAE 



Genus Skeletonema Greville 

 Greville, 1865 



Skeletonema costatum (Greville) Cleve. 



Cleve, 1878, p. 18. 

 Lebour, 1930, p. 70, fig. 43. 

 Hustedt, 1928, p. 311, fig. 149. 

 Melosira costata Greville, 1866 b, p. 77. 



Cells discoid, oblong or weakly spherical. Valves mostly convex, but sometimes 

 almost flat. Cells united to form filaments by means of a marginal ring of long spines ; 

 spines straight, filaments straight, sometimes slightly spiral, weakly siliceous. Owing 

 to the length of the spines the spaces between the individual cells are frequently larger 

 than the cells themselves. The striation of the valves is extremely fine, if anything it is a 

 little more pronounced upon the peripheral zone or valve mantle ; but on the face of the 

 valve it is almost invisible. Chromatophores: usually two small plates. Diameter of 

 valve 8-15/x ; pervalvar axis 4-12/x. Length of spines usually 6-10/x. 



A very common pelagic diatom, frequently associated with a coastal flora, when 

 present it is usually found in large numbers. 



Observed at Sts. WS 593, 594, 598, 600, 601, 602, 644, 647, 649, 705, 709, 710. 



Genus Stephanopyxis Ehrenberg 

 Ehrenberg, 1844 



Stephanopyxis Palmeriana (Greville) Grunow. 



Grunow, 1884, p. 90. 

 Lebour, 1930, p. 74, fig. 47. 



