SYSTEMATIC ACCOUNT 237 



Hustedt, 1928, p. 308, fig. 147. 



Creswellia Palmeriana Greville, 1865, p. 2, pi. 1, fig. 9. 



Cells shortly cylindrical, oblong, forming long chains. Valves circular, convex, 

 valve mantle or peripheral zone deep. Valve covered with hexagonal areolation, areoles 

 large at the centre of the valve face, becoming increasingly smaller through the peri- 

 pheral zone as they approach the valve edge. The two valves of the frustule adhere 

 tightly by their edges. Valves furnished with a ring of stout spines; through each runs 

 a fine, slightly spiral canal by means of which cytoplasmic continuity is maintained from 

 cell to cell. Chromatophores : numerous small plates. Diameter of valve 70-1 10/x; 

 pervalvar axis 85/x. 



A pelagic diatom, favouring warm water of high salinity, common in Indian Ocean, 

 particularly around South Africa. Found occasionally in the Mediterranean. 



Observed at Sts. 425, 427, 428, 433, 434, 1373, 1575. 



Stephanopyxis turris (Greville) Ralfs ex Pritchard. 



Pritchard, 1861, p. 826, pi. 5, fig. 74. 



Karsten, 1905, p. 73, pi. 2, fig. 1. 



Lebour, 1930, p. 73, figs. 45-46. 



Hustedt, 1928, p. 304, fig. 140. 



Cresivellia Turris Greville ex Gregory, 1857, p. 538, pi. 14, fig. 109. 



Cells cylindrical, oblong, with rounded ends, united to form chains. Valves circular, 

 strongly convex, peripheral zone very deep. Valve covered with strong and irregular 

 areolation, areoles usually the same size on all parts of the valve. Peripheral zone or 

 mantle usually longer than diameter of the valve. Valve frequently constricted in zonal 

 view above point of suture, giving the valve a flanged or helmet appearance. Valve 

 furnished with a concentric ring of stout spines as in S. Palmeriana. Chromatophores : 

 numerous small plates. Diameter of valve 20-60/x, mostly 52/x ; pervalvar axis 40-90^. 



A pelagic diatom frequent in temperate seas, common off the Atlantic and Pacific 

 coasts of America, rare in polar waters. Frequently found in tertiary fossil deposits both 

 in New Zealand and Europe. 



Observed at Sts. 261, 262, 435, 437, 722, 723 ; WS 593, 594, 647, 650, 709, 710, 712. 



Subfamily THALASSIOSIROIDEAE 



Genus Thalassiosira Cleve 

 Cleve, 1873 



Thalassiosira antarctica Comber. 

 Comber, 1896, p. 491, pi. n. 



Cells discoid, united to form chains composed of three to twenty cells, or dense 

 mucilaginous colonies. Valves circular, surface slightly convex. Valves furnished with 

 somewhat coarse puncta, arranged in radiating and frequently bifurcate rows ; central 

 granules usually larger, sometimes spinous. Margin of valve furnished with numerous 

 spinulae, small, often arranged in two irregular rows. Resting spores frequently ob- 



