244 DISCOVERY REPORTS 



polygonal ; outside the central rosette for a short distance along the radius the areoles 

 are somewhat small, becoming larger, until at a distance equal to half the radius, they 

 attain their maximum size after which they again decrease. Areoles furnished with 

 characteristic secondary and tertiary structure. Chromatophores : numerous large 

 rounded plates. Diameter of valve 230-360^, mostly 330/x. 



It is remarkable that this diatom was observed only in the material from the Pacific 

 Ocean. It has a wide distribution in almost all temperate seas, but seldom occurs in 

 great numbers. A neritic diatom, favouring a fairly high salinity. 



Observed at Sts. WS 622, 623, 644, 645, 666. 



Coscinodiscus beta Karsten. 



Karsten, 1907, p. 362, pi. 36, fig. 1. 



Cells discoid, small, solitary, almost flat but sometimes weakly convex. Valves covered 

 with fine radial punctation. Margin of valve furnished with a circlet of short erect 

 spines or horns. Chromatophores: small oval or roundish plates, few in number. 

 Diameter of valve 92-100/u. 



This form was first described by Karsten from Indian Ocean material collected in 

 tropical waters. The specimens here described were collected to the south of Mada- 

 gascar and appeared to be slightly larger and more robust than the type specimen. The 

 species is not common and has not been recorded outside the Indian Ocean. It is never 

 found in large numbers. 



Observed at St. 440. 



Coscinodiscus bouvet Karsten. (PL XIII, figs. 3, 4.) 



Karsten, 1905, p. 83, pi. 3, fig. 9. 

 Heiden and Kolbe, 1928, p. 495. 

 Mangin, 1915, p. 52, fig. 36. 



Cells circular in valve view, octagonal in girdle view, medium to large, sometimes 

 solitary but usually in short chains of two to four frustules. The formation of the valve 

 is characteristic and somewhat peculiar. The central area of the valve, equal to about 

 one-third of the total diameter, but often less, is flat or nearly so ; the peripheral area 

 usually falls sharply down to the girdle, producing a characteristic octagonal pervalvar 

 section. Sometimes the peripheral area falls in an undulating manner. Often the central 

 areas of the two valves of the same frustule are of different diameters, that is one valve 

 of the frustule may be more deeply conical than the other. Surface of valve covered 

 with a fine hexagonal areolation ; central rosette of cells regular and strongly marked ; 

 the areoles decrease in size as they approach the margin of the valve. Valve margin 

 furnished with small spinulae. Girdle broad, usually bearing a number of lines. 

 Chromatophores : numerous irregular stellate bodies, usually confined to the centre of 

 the frustule or lying along the girdle. Diameter of valve 140-200^ (total), 40-70/x (central 

 area); pervalvar axis 144-180^. 



This very handsome diatom is characteristic of Antarctic waters, and has a wide dis- 

 tribution in the southern seas. It is commonly met with around South Georgia, the 



